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THE  UNIVERSITY  OF  CHICAGO 
SCIENCE  SERIES 


Editorial  Committee 

ELIAKIM  HASTINGS  MOORE,  Chairman 

JOHN  MERLE  COULTER 

ROBERT  ANDREWS  MILLIKAN 


The  University  of  Chicago  Science  Series, 
established  by  the  Trustees  of  the  University, 
owes  its  origin  to  a  feeling  that  there  should  be 
a  medium  of  publication  occupying  a  position 
between  the  technical  journals  with  their 
short  articles  and  the  elaborate  treatises  which 
attempt  to  cover  several  or  all  aspects  of  a 
wide  field.  The  volumes  of  the  series  will 
differ  from  the  discussions  generally  appearing 
in  technical  journals  in  that  they  will  present 
the  complete  results  of  an  experiment  or  series 
of  investigations  which  previously  have  appeared 
only  in  scattered  articles,  if  published  at  all. 
On  the  other  hand,  they  will  differ  from  detailed 
treatises  by  confining  themselves  to  specific 
problems  of  current  interest  and  in  presenting 
the  subject  in  as  summary  a  manner  and  with 
as  little  technical  detail  as  is  consistent  with 
sound  method. 


THE  BIOLOGY  OF  TWINS 

(MAMMALS) 


THE  CNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO.  ILLINOIS 


Brents 

THE  BAKEE  &  TAYLOR  COMPANY 

NEW  YORK 

THE  CUNNINGHAM,  CURTISS  &  WELCH  COMPANY 

LOS  ANGELES 


THE  CAMBRIDGE  UNIVERSITY  PRESS 

LONDON  AND  EDINBUROH 

THE  MARUZEN-KABUSHIKI-KAISHA 

TOmrO,   OSAKA,    KYOTO,   FtJKUOKA,   SENDAI 

THE  MISSION  BOOK  COMPANY 

SHANGHAI 

KARLW.  HIERSEMANN 

LEIPZIS 


THE 

BIOLOGY  OF  TWINS 

(MAMMALS) 


By 


HORATIO   HACKETT  NEWMAN 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


Copyright  1917  By 
The  University  of  Chicago 


All  Rights  Reserved 


Published  March  1917 


Composed  and  Printed  By 

The  University  of  Chicago  Press 

Chicago,  Illinois,  U.S.A. 


PREFACE 

The  present  volume  brings   together  for   the  first 

time   a   considerable   mass   of   data   dealing   with   the 

phenomenon   of   twins   in   man   and   other   mammals. 

Twins  are  so  inherently  interesting  to  so  many  people 

that  it  is  hoped  by  the  writer  that  some  light  on  how 

twins  ''happen"  will  be  welcomed  by  the  general  reader 

as  well  as  by  the  biologist.     There  are  many  thoroughly 

interesting    and    nontechnical    phases    of    twin-biology 

that  will  appeal  to  anyone  who  is  a  twin  or  has  personal 

acquaintance  with  twins.     There  must  be  certain  other 

phase^/of   the   subject,   however,   that   are   largely   of 

value  to  the  professional  biologist.     It  has  been  the  aim 

of  this  book  to  satisfy  both  the  general  and  the  technical 

reader    without    sacrificing    unduly    the    demands    of 

simplicity^n  the  one  hand  or  of  scientific  adequacy  on 

the  other. 

H.  H.  N. 


Vll 


CONTENTS 

PAGE 

Introduction i 

CHAPTER 

I.  Various  Kinds  of  Human  Twins       .'....  8 

II.  Twinning  (Polyembryony)  in  the  Armadillo  (Dasy- 

pus  novemcinctus) .  25 

III.  Modes  or  Twinning  in  Other  Species  of  Armadillo  68 

IV.  Theories     of    Polyembryonic    Development    in 

Dasypus 85 

V.  Twinning  in  Ruminants — ^The  Freemartin      .     .  95 

VI.  Twins  in  Relation  to  General  Biological  Prob- 
lems        no 

VII.  Variation  and  Heredity  in  Twins 125 

Index        181 


IX 


INTRODUCTION 

Everyone  is  or  should  be  interested  in  twins.  It 
is  my  task  to  bring  together  the  facts  about  twins  and 
to  show  the  bearings  of  these  facts  on  fundamental 
problems  of  biology.  It  would  be  an  easy  task  to  write 
a  treatise  on  twins  for  the  expert  embryologist  or  student 
of  genetics,  but  it  is  far  less  easy  to  present  this  material 
adequately  and  to  make  it  crystal-clear  to  those  who 
are  not  specialists.  It  has  seemed  necessary  to  strike 
a  compromise  between  a  technical  presentation  and 
a  somewhat  popular  treatment  of  the  subject.  Much 
of  the  more  general  matter  in  all  of  the  chapters  will  be 
found  available  for  the  general  reader,  but  some  of  the 
descriptive  embryology,  which  is  the  foundation  of  our 
special  knowledge,  will  be  rather  difficult  even  to  the 
embryologist.  Every  effort  has  been  made  to  simplify 
this  part  of  the  book  without  running  the  risk  of  denatur- 
izing  it.  Again,  there  are  parts  of  the  chapters  on 
heredity  that  will  appeal  especially  to  students  of  that 
important  subject,  but  will  have  only  a  minor  appeal 
to  the  general  reader. 

It  is  impossible  to  avoid  technical  terms,  especially 
in  descriptive  embryology,  but  where  certain  simple 
terms  serve  as  well  as  the  more  technical  ones  they  will 
be  used.  In  referring  to  early  embryos  of  human  beings 
or  of  armadillos  we  might  use  the  words  ^'blastodermic 
vesicle"  or  "blastocyst,"  but  it  will  be  simpler  to  use 
the  common  word  ''egg"  for  the  early  mammalian 
embryo   and   its   membranes.     Again,   in   speaking   of 

N.  c.  Stefe  Ceifer 


2  THE  BIOLOGY  OF  TWINS 

the  sex  of  twins  we  shall  avoid  for  more  reasons  than 
one  the  terms  "homosexual"'  and  "heterosexual," 
which  refer  respectively  to  twins  both  of  the  same  sex 
and  twins  of  opposite  sexes,  and  shall  use  the  less 
objectionable  terms  "same-sexed"  and  "opposite- 
sexed." 

Where  the  use  of  technical  terms  appears  unavoid- 
able, the  reader  will  find  that,  as  a  rule,  a  term  is  defined 
when  first  used  and  possibly  in  more  places  than  one. 
Frequently,  too,  when  the  verbal  description  is  difficult 
of  comprehension  the  illustrations  will  give  the  essential 
information. 

In  this  book  an  attempt  is  made  to  gather  from  many 
sources  the  facts  about  mammalian^  twins  and  to  unify 
these  varied  situations  into  one  point  of  view.  My 
own  interest  in  this  subject  has  grown  out  of  eight 
years'  study  of  what  is  perhaps  the  most  striking  case 
of  twin-production  known:  that  exhibited  by  the  nine- 
banded  armadillo  of  Texas  (Dasypus  novemcinctus) . 
The  somewhat  disproportionate  space  devoted  to  the 
phenomenon  of  polyembryony  in  this  species  needs  no 
apology.  The  various  aspects  of  its  biology  have  been 
more  extensively  studied  than  those  of  any  other 
species,  and  an  author  may  be  forgiven  for  emphasizing 
the  parts  of  his  subject  with  which  he  has  a  first-hand 
acquaintance. 

^  The  term  "homosexual"  is  extensively  used  in  the  literature 
dealing  with  abnormal  sex  relations  and  is  therefore  pre-empted. 

*  An  extensive  chapter  on  twinning  among  the  various  vertebrate 
classes  below  the  mammals  would  appear  to  lend  completeness  to  this 
volume,  but  a  review  of  the  extensive  literature  convinces  me  that  such 
a  chapter  would  be  more  confusing  than  helpful  to  the  general  reader. 
I  shall  therefore  deal  with  twinning  in  mammals  only. 


INTRODUCTION  3 

No  simple  definition  of  twinning  can  be  given,  for 
the  term  has  come  to  be  appHed  to  at  least  four  distinct 
situations : 

1.  The  production  from  plural  eggs  (zygotes)  of 
plural  offspring  in  species  that  habitually  produce  but 
a  single  offspring  at  a  birth.  This  would  include  some 
twins  in  man,  as  well  as  triplets,  quadruplets,  and  larger 
sets  of  simultaneously  born  offspring.  Cattle  and  sheep 
twins  and  triplets  also  belong  to  this  category. 

2.  The  production  of  plural  offspring  either  sporadi- 
cally or  as  a  specific  character,  from  a  single  fertilized 
egg  (zygote).  Such  twins,  quadruplets,  or  larger  sets 
of  offspring  are  known  as  monozygotic,  and  this  mode 
of  reproduction  is  known  as  polyemhryony.  Two  spe- 
cies of  armadillo  belonging  to  the  genus  Dasypus  ex- 
hibit specific  polyembryony;  also  there  appears  to  be 
sporadic  polyembryony  in  man  and  possibly  in  other 
species. 

3.  The  habitual  or  specific  production  of  paired 
offspring,  each  of  which  is  derived  from  one  fertilized 
egg.  Such  dizygotic  twins  are  specific  for  the  armadillo 
Euphractus  villosus,  and  possibly  for  other  species  of 
that  genus. 

4.  The  sporadic  production  of  conjoined  twins  and 
various  types  of  double  monsters.  This  is  the  category 
to  which  Siamese  twins  and  similar  monstrosities  belong; 
the  condition  is  known  to  occur  in  many  mammalian 
species,  although  very  little  study  of  the  phenomenon 
has  been  made  except  for  man.  Some  .conjoined  twins 
are  evidently  monozygotic  and  are  due  to  processes  akin 
to  polyembryony,  but  others  are  evidently  due  to  a 
secondary  fusion  of  dizygotic  individuals. 


4  THE  BIOLOGY  OF  TWINS 

The  one  fact  that  stands  out  above  all  others  in  this 
connection  is  that  there  are  really  but  two  distinct 
kinds  of  twins:  those  that  come  from  a  single  egg  and 
those  that  come  from  two  or  more  eggs.  The  former 
type  involves  some  process  of  fission  or  division  of  a 
germ,  which  is  at  first  a  single  individual,  into  two  or 
more  complete  or,  in  certain  cases  of  incomplete  twin- 
ning, partial  individuals.  This  is  really  the  only  true 
type  of  twinning. 

When,  however,  twins  or  triplets,  etc.,  are  derived 
from  two  or  more  eggs,  the  biology  of  the  situation 
differs  very  little  from  the  ordinary  phenomenon  of 
multiple  births,  as  seen  in  swine,  dogs,  cats,  and  other 
common  mammals.  In  these  animals  we  do  not  use  the 
term  twin,  triplet,  or  quadruplet,  because  this  giving 
birth  to  a  number  of  offspring  at  a  time  is  the  normal 
condition,  whereas  a  single  offspring  is  exceptional.  It 
is  only  in  those  species  that  normally  give  birth  to  but 
one  offspring  at  a  time  that  we  note  especially  the  more 
or  less  frequent  exceptions  to  the  rule,  and  refer  to 
double  births  as  twins,  triple  births  as  triplets,  quadruple 
births  as  quadruplets,  etc. 

Monozygotic  twinning,  where  a  single  egg  produces 
plural  offspring,  is  therefore  a  phenomenon  that  should 
be  considered  as  only  a  phase  of  the  much  more  general 
phenomenon  of  symmetrical  division.  The  develop- 
ment of  the  right-  and  left-hand  homologous  organs 
in  a  bilateral  organism  is  essentially  a  twinning  process, 
for  it  involves  the  division  of  a  median  unpaired  pri- 
mordium  into  two  equivalent  parts,  one  of  which  is 
the  mirror-image  of  the  other.  In  many  cases  this 
twinning   of  parts   may   be   more   or   less   completely 


INTRODUCTION  5 

inhibited,  so  that  a  failure  of  certain  parts  to  divide 
occurs  and  a  single  median  structure  appears.  The 
paired  eyes,  for  example,  may  fail  to  develop  and  a 
single  median  cyclopic  eye  may  result.  Just  as  there 
may  be  an  inhibition  of  the  normal  culmination  of  the 
process  of  bilateral  division,  so  there  is  frequently  an 
excess  of  division  resulting  in  two  bilateral  structures 
becoming  completely  separated,  as  when  a  single 
individual  develops  two  heads  or  two  tails,  while 
the  remainder  is  a  more  or  less  normal  individual. 
The  whole  matter  of  bilateral  development  appears 
to  be  quantitative  in  nature,  in  that  the  same 
type  of  process  may  go  not  so  far  or  farther  than 
normal. 

There  are  known  for  man  as  well  as  for  the  lower 
vertebrates  all  stages  of  twinning,  ranging  from  incom- 
pletely bilateral  forms  that  are  subnormal,  such  as 
cyclopic  monsters,  through  various  grades  of  super- 
normal forms,  such  as  two-headed  types,  double  mon- 
sters, and  conjoined  twins,  culminating  in  completely 
separate  monozygotic  or  duplicate  twins.  That  all 
these  types  are  merely  phases  of  the  same  process  of 
bilateral  doubling  is,  I  believe,  beyond  question,  as  will 
be  shown  in  the  chapters  that  follow. 

The  phenomenon  of  twinning  then  will  be  seen  to 
be  a  very  fundamental  process,  one  almost  universal 
in  the  field  of  biology.  For  wherever  we  have  bilateral 
doubling  we  have  twinning  in  some  form. 

All  expressions  of  the  twinning  process  involve  the 
same  biological  problems:  those  of  symmetry,  heredity, 
and  sex;  but  perhaps  the  process  of  duplicate  twin- 
ning proper,  where  two  or  more  completely  separate 


6  THE  BIOLOGY  OF  TWINS 

individuals  are  produced,  affords  the  most  available 
material  for  a  study  of  these  problems. 

The  problems  of  twinning  and  those  of  sex  and  of 
heredity  are  inextricably  interwoven;  each  of  them 
appears  to  be  a  corollary  of  the  other.  It  is  not  surpris- 
ing then  that  some  of  the  most  conclusive  evidence  as 
to  the  nature  and  mode  of  sex-determination  and  much 
new  light  on  the  nature  and  limits  of  hereditary  control 
come  from  a  study  of  twins. 

A  collection  of  data  upon  twinning  in  mammals 
brings  together  more  biological  curiosities  than  is  fur- 
nished by  any  other  field  of  similar  scope  with  which  I 
am  acquainted.  The  armadillos  furnish  two  strangely 
unique  situations  quite  opposite  in  character.  In  Das- 
ypus  there  is  the  sphtting  up  of  a  single  egg  into  a 
number  of  separate  embryos,  while  in  Euphractus  two 
originally  separate  eggs  secondarily  undergo  extensive 
fusion  of  their  membranes  so  as  to  produce  mono- 
choriaP  twins  quite  deceptively  Hke  those  known  to  be 
monozygotic.  Twinning  in  cattle  involves  the  very  odd 
phenomenon  of  the  freemartm,  where  usually  a  female 
born  co-twin  to  a  male  is  sterile  and  shows  certain 
male  characteristics.  An  analysis  of  this  peculiar  situa- 
tion goes  far  toward  clearing  up  the  problems  of  the 
nature  of  sex  and  of  the  factors  which  control  it. 

There  are  also  many  strange  facts  about  the  va- 
rious kinds  of  human  twins.  DupHcate  or  identical 
twins  are  of  unusual  interest;  conjoined  twins  of  the 

^  Monochorial  twins  are  surrounded  by  a  single  chorionic  mem- 
brane. Usually  a  single  chorion  covers  but  one  embryo  and  comes 
from  a  single  egg;  hence  it  is  customarily  assumed  that  the  monocho- 
rial condition  implies  a  monozygotic  origin. 


INTRODUCTION  7 

Siamese  type  and  so-called  '' parasite"  twins  are  also 
oddities  that  have  long  been  known  but  little  under- 
stood. Owing  to  the  special  interest  that  naturally 
attaches  to  human  affairs,  the  first  chapter  will  be 
devoted  to  human  twins,  although  so  little  is  known 
about  their  mode  of  development.  The  second  chapter 
gives  a  full  account  of  the  process  of  polyembryonic 
twinning  in  the  armadillo,  Dasypus  novemcinctus ,  and 
there  is  reason  to  believe  that  in  this  material  we  have 
the  only  key  to  the  mechanics  of  human  twinning. 

I  wish  to  express  my  thanks  to  my  friends  W.  H. 
Osgoode  and  F.  R.  Lillie  for  reading  the  manuscript 
and  for  useful  suggestions;  to  Mr.  Kenji  Toda  for  his 
aid  in  preparing  illustrations;  and  to  those  authors 
from  whom  figures  have  been  borrowed. 


CHAPTER  I 
VARIOUS  KINDS  OF  HUMAN  TWINS 

Human  twins  have  always  been  objects  of  especial 
interest,  partly  perhaps  on  account  of  the  fine  humor 
of  the  situation,  and  partly  because  of  the  frequent 
occurrence  of  ''look-alike"  twins  or  "duplicate"  twins. 
Popular  interest  has  quite  naturally  focused  upon  the 
striking  similarity,  amounting  in  some  cases  to  almost 
complete  identity,  which  exists  in  certain  types  of  twins; 
this  emphasis  upon  resemblance  is  justified  by  the  bio- 
logical analysis  of  twinning,  as  is  shown  in  what  follows. 

Biologists  have  for  some  time  recognized  at  least  two 
distinct  types  of  human  twins:  fraternal  and  duplicate. 
Fraternal  twins  may  or  may  not  be  same-sexed,  are 
usually  no  more  alike  than  are  brothers  and  sisters,  and 
are  believed  to  be  dizygotic,  derived  from  two  fertihzed 
eggs.  Duplicate  or  identical  twins  are  always  of  the 
same  sex,  are  almost  identical,  and  are  believed  to  be 
monozygotic,  derived  from  a  single  fertilized  egg. 

No  twins  occur  in  nature  at  all  comparable  with  the 
old  favorite  type  which  has  done  such  signal  service  in 
the  drama  and  in  fiction  (not  to  mention  the  ''movies"), 
wherein  brother  and  sister  are  identical.  Science, 
however,  can  afford  to  be  magnanimous,  so  let  us  as  a 
concession  to  art  include  in  our  list  these  "literary 
twins. " 

Another  type  of  human  twins,  stranger  than  fiction, 
is  found  in  conjoined  twins  and  double  monsters,  of 

8 


VARIOUS  KINDS  OF  HUMAN  TWINS  9 

which  the  Siamese  twins  furnish  a  stock  example.  There 
appears  to  be  a  graded  series  of  types  ranging  between 
duplicate  twins  and  the  less  monstrous  types  of  con- 
joined twins;  similarly,  the  conjoined  twins  appear  to 
grade  into  the  various  kinds  of  double  monsters.  Some 
conjoined  twins  are  very  lightly  connected  and  some 
double  monsters  are  so  closely  united  that  one  individ- 
ual may  be  a  mere  degenerate  parasite  upon  the  other. 
The  fact  that  very  lightly  conjoined  twins  exist  would 
point  to  the  probabiUty  that  such  twins  might  some- 
times be  born  separately.  This  is  Wilder's  view  of  the 
relation  between  dupKcate  twins  and  double  monsters. 
Lightly  conjoined  twins  are  always  of  the  same  sex  and 
are  strikingly  similar;  there  seems  to  be  no  question 
as  to  their  monozygotic  origin.  What  more  natural, 
therefore,  than  to  infer  that  separate  twins  which  are 
of  the  same  sex  and  strikingly  alike  are  also  monozygotic  ? 
Considerable  direct  and  indirect  evidence  that 
monozygotic  twins  are  of  frequent  occurrence  in  man 
is  available.  Perhaps  the  most  conclusive  evidence  for 
this  idea  is  to  be  found  in  a  study  of  the  sex-ratios 
of  twins.  Data  are  available  from  several  different 
sources,  but  perhaps  the  best  is  that  given  by  Nichols,^ 
which  is  herewith  presented: 


Sex  of  Twins 

s$     • 

Si 

?2 

Frequency  of  occurrence .... 

234,497 

264,098 

219,312 

'  J.  B.  Nichols,  Memoirs  of  the  American  Anthropological  Associa- 
tion, I  (1907). 


lo  THE  BIOLOGY  OF  TWINS 

This  is  approximately  a  ratio  of  i  :  ^  •  ^i^  • 
Now  if  all  twin  births  in  man  are  dizygotic  and  the  sex 
is  predetermined  at  the  time  of  fertilization,  as  there 
is  every  reason  to  believe,  the  sex-ratios  of  twins  should 
be:  J  :  2  *  I  •  There  are,  however,  nearly  twice 
as  many  same-sexed  twins  as  there  should  be  on  this 
basis,  and  the  only  satisfactory  explanation  of  this 
discrepancy  between  the  observed  and  the  expected 
ratios  appears  to  be  that  nearly  half  of  all  same-sexed 
twins  are  monozygotic  and  hence  morphologically  stand 
for  hut  one  individual  to  the  pair.  It  would  appear 
then  that  in  Nichols'  data  the  difference  between  the 
actual  numbers  of  same-sexed  twins  and  half  the  number 
of  opposite-sexed  twins  will  give  the  probable  number  of 
monozygotic  twins  of  each  sex;  this  would  be  102,448 
monozygotic  male  twins  and  87,263  monozygotic  female 
twins.  About  one-fourth  of  all  human  twins  then,  if 
this  reasoning  holds,  are  monozygotic.  My  own  observa- 
tion and  that  of  biologists  with  whom  I  have  discussed 
this  point  agree  very  closely  with  this  conclusion. 

The  form  of  the  human  uterus  and  the  intra-uterine 
relations  of  twins  serve  as  another  line  of  evidence 
favoring  the  existence  of  monozygotic  human  twins. 
The  human  uterus  is  of  the  simple  type,  resembling 
that  of  the  armadillo  Dasypus,  and  is  not  adapted  for 
ordinary  multiple  gestation.  Such  a  uterus,  however, 
is  apparently  as  favorable  for  polyembryony  (the 
production  of  plural  embryos  from  one  egg)  as  that  of 
our  armadillos,  in  which  this  kind  of  reproduction 
occurs  normally. 

The  evidence  furnished  by  the  data  collected  by 
obstetricians  of  twins  in  utero  also  favors  the  existence 


VARIOUS  KINDS  OF  HUMAN  TWINS  ii 

of  monozygotic  twins.  Frequently  this  evidence  is  lack- 
ing in  very  essential  points.  Sometimes,  for  example,  the 
sex  is  not  mentioned,  and  never,  so  far  as  I  am  aware,  is 
there  information  about  the  number  of  corpora  lutea  pres- 
ent. The  importance  of  the  latter  data  cannot  be  over- 
emphasized in  this  connection;  a  knowledge  of  whether 
one  or  more  corpora  lutea  are  present  would  furnish  a 
crucial  test  of  the  number  of  ova  concerned  in  a  given 
pregnancy.  In  not  a  single  case  of  human  multiple 
births,  so  far  as  I  am  aware,  has  the  number  of  corpora 
lutea  been  noted.  The  reason  for  this  is  obvious:  to 
secure  this  information  would  require  an  operation  dur- 
ing pregnancy.  It  seems  quite  probable,  however,  that 
post-mortem  examinations  of  pregnant  women,  and  of 
those  dying  after  abdominal  operations  or  during  par- 
turition, might  serve  to  give  occasionally  just  the  kind 
of  information  that  we  must  have  in  order  to  prove  the 
existence  of  monozygotic  twinning  in  man.  I  would 
therefore  urge  upon  surgeons  and  obstetricians  the  im- 
portance of  collecting  information  as  to  the  corpora 
lutea  whenever  opportunity  arises. 

Although  data  as  to  intra-uterine  relations  are  incom- 
plete and  inconclusive,  they  form  the  only  really  direct 
evidence  now  available  on  the  mode  of  twinning  in  man. 
The  most  reliable  collection  of  such  data  is  that  of 
0.  Schultze.^  He  grouped  his  types  into  four  catego- 
ries which  have  been  given  by  Wilder,''  together  with 
the  latter's  comments,  as  follows: 

Case  I. — Two  separate  blastodermic  vesicles  with  two  decid- 
uous reflexae  and  two  placentae.    This  case  is  probably  one  in 

^  O.  Schultze,  Leipzig,  1897. 

'H.  H.  Wilder,  American  Journal  of  Anatomy,  III  (1904). 


12  THE  BIOLOGY  OF  TWINS 

which  there  are  two  separate  eggs,  either  from  the  same  or  from 
opposite  oviducts,  and  implanted  at  some  little  distance  from  one 
another.  In  one  case  investigated  by  Von  KoUiker,  the  two 
deciduae  were  distinct  but  partially  adherent  over  the  surfaces 
in  mutual  contact,  and  in  another  the  contact  surfaces  had  fused 
into  a  single  wall  into  which,  from  the  two  opposite  sides,  the 
chorionic  viUi  of  the  two  embryos  had  grown.  In  addition  to 
this,  one  of  the  placentae  was  of  the  type  known  as  placenta 
marginata,  caused  by  a  fold  of  the  decidua.  (This  is  evidently 
a  normal  multiple  birth,  a  condition  hard  to  accomplish  in  a 
uterus  of  the  shape  found  in  human  beings,  and  often  attended 
by  such  phenomena  as  adhesions,  fusions,  and  foldings,  all 
indicative  of  crowding  and  nothing  else.^-Wilder.) 

Case  II. — Two  separate  blastodermic  vesicles  inclosed  in  a 
single  decidua.  Placentae  fused  with  one  another  but  with  two 
separate  sets  of  umbilical  vessels.  Two  chorions  fused  at  the 
point  of  contact.  This  case  is  more  frequent  than  (I)  but  appar- 
ently results  from  the  same  general  cause,  i.e.,  two  separate  eggs, 
which  are,  however,  implanted  nearer  together.  This  w^ould 
seem  more  likely  to  happen  if  both  eggs  came  from  the  same  side. 
(The  conditions  are  seen  to  be  similar  to  those  of  (I),  the  greater 
degree  of  fusion  being  well  accounted  for  by  the  approximation 
of  the  two  eggs  to  one  another. — Wilder.) 

Case  III. — Two  amnions  and  two  umbilical  cords  with  a 
single  placenta  in  the  middle  of  which  the  two  cords  meet  and 
ujKjn  which  the  umbilical  vessels  closely  anastomose.  These  are 
inclosed  in  a  single  chorion  and  covered  with  a  single  decidua 
reflexa.  This  case  is  said  by  Hyrtl  to  be  more  frequent  than 
(I)  and  (II),  but  not  as  frequent,  according  to  Spath.  The  twins 
are  always  of  the  same  sex.  Schultze  says  that  the  explanation 
of  this  singular  condition  is  zweijelhajt  and  gives  the  following 
possible  explanations:  (i)  At  first  two  chorions,  as  in  (II),  the 
contact  wall  between  which  becomes  absorbed  later;  (2)  may 
have  come  from  a  single  egg  with  a  double  yolk,  or  (3)  from  an 
ovarial  egg  with  two  nuclei  (cf.  Franque,  1898,  Stoeckel,  1899,. 
H.  Rabl,  1899,  and  von  Schuhmacher  u.  Schwarz,  1900).  It  is 
conceivable  that  from  such  an  egg  as  this  last,  two  blastodermic 
vesicles  and  two  chorions  could  develop  within  one  zona  pellucida, 


VARIOUS  KINDS  OF  HUMAN  TWINS  13 

at  a  later  stage  of  which  two  chorions  could  fuse.  Von  KoUiker 
considers  it  more  probable,  however,  that  in  such  a  case  the  egg 
would  develop  two  embryonic  areas  upon  a  single  blastodermic 
vesicle  and  that  a  single  chorion  would  then  be  the  natural  result. 
Each  embryonic  area  would  develop  its  own  amnion.  In  this 
case  the  two  allantois  would  necessarily  fuse,  being  included 
in  a  single  chorion,  and  there  would  come  to  be  between  the  two 
embryos  a  single  (common)  yolk  sac  with  two  yolk  stalks.  Von 
Kolliker  has  observed  such  cases  in  hen's  eggs  (but  without  fusion 
of  the  allantoids).  M.  Braun  has  seen  it  in  lizards,  and  Panum 
describes  separate  embryonal  areas  upon  one  yolk  (hen's  egg). 
See  also  Koestner's  figure  of  a  double  egg  of  PrisHurus,  1898. 
(This  case  seems  to  put  us  on  the  right  track  regarding  the  origin 
of  duplicate  twins,  especially  since  it  is  stated  that  the  twins  are 
always  of  the  same  sex,  and  although  observations  of  later  physical 
identity  are  wanting,  it  seems  safe  to  assume  it.  It  would  seem 
highly  improbable,  however,  that  duplicate  twins  would  arise 
from  an  ovarian  egg  with  two  nuclei,  since  in  such  case  the  fertili- 
zation could  be  effected  only  by  means  of  two  spermatozoa,  thus 
introducing  two  paternal  characters;  but  if  we  reject  all  of 
Schultze's  alternatives  and  substitute  the  possibility  suggested 
above,  that  of  the  complete  separation  of  the  two  blastomeres 
resulting  from  the  first  cleavage  of  the  fertilized  egg,  the  two 
components  would  still  remain  within  one  zona  pellucida  and 
would  later  become  inclosed  within  a  single  chorion,  which  would 
develop  a  single  placenta  to  which  each  allantois  would  become 
later  attached.  Each  blastomere  would  undoubtedly  form  at 
first  an  independent  blastodermic  vesicle,  but  the  close  association 
of  the  two  would  readily  tend  toward  a  fusion  of  the  contact 
surfaces,  thus  forming  a  single  vesicle  upon  the  surface  of  which 
are  two  embryonic  areas.  If  far  enough  apart  from  one  another, 
each  would  develop  its  own  amnion,  but  if  near  together  a  com- 
mon amnion  would  result,  thus  producing  the  condition  given  in 
Case  IV.  The  whole  matter  of  the  actual  condition  of  the  de- 
velopment of  the  two  associated  embryos  is  very  obscure,  as 
there  are  but  scattered  and  insuflicient  data  bearing  upon 
the  case.  It  will  receive  a  more  extended  consideration  later 
on,  under  the  heading  "Origin   of   Composite   IMonsters"  and 


14  THE  BIOLOGY  OF  TWINS 

''Other  Recent  Theories  Concerning  the  Genesis  of  Composite 
Monsters. " — Wilder.) 

Case  IV. — Similar  to  (III),  but  with  both  embryos  inclosed 
in  a  single  amnion.  This  is  a  very  rare  case,  explicable  only  by 
postulating  a  single  blastodermic  vesicle  upon  which  the  two 
embryonal  areas  are  nearly  or  entirely  in  contact  with  one  another, 
a  case  which  has  been  described  by  several  authors  as  occurring 
in  the  hen's  egg.  In  such  a  case  there  would  be  an  almost  irre- 
sistible tendency  toward  the  fusion  of  the  two  embryos  along 
the  line  of  mutual  contact,  thus  producing  some  form  of  composite 
monster.  [Schultze  says  Doppelmissbildungen,  but  I  use  the 
word  double  in  a  more  restricted  sense,  as  explained  below.] 

(As  Case  II  is  seen  to  be  a  variation  of  Case  I  with  the  two 
enibryos  nearer  together,  so  Case  IV  is  seen  to  be  a  similar  varia- 
tion of  Case  III,  with  a  similar  result,  i.e.,  the  more  complete 
fusion  of  the  parts,  although  here,  owing  to  the  direct  connection 
of  the  two  embryos,  the  fusion  is  liable  to  extend  also  to  these 
and  produce  abnormal  results.  There  are  thus  primarily  not 
four,  but  two  cases,  corresponding  to  the  two  types  of  twins, 
fraternal  and  duplicate.  The  close  connection  of  IV  and  III 
suggests  what  may  have  already  occurred  to  the  reader:  that 
many  cases  of  compound  monsters  come  under  the  same  category 
as  separate  duplicates.  This  is  quite  probable,  but  such  forms, 
arising  from  a  secondary  fusion,  would  be  more  asymmetrical  and 
more  or  less  unequal,  and  would  come  under  the  class  of  autosite 
and  parasite  rather  than  that  of  symmetrical,  or  genuine  double 
monsters. — Wilder.) 

This  rather  extensive  quotation  from  Wilder's  mon- 
ograph on  Duplicate  Twins  and  Double  Monsters  is 
given  largely  to  show  the  inadequacy  of  the  embryo- 
logical  data  upon  which  conclusions  as  to  the  mode  of 
origin  of  twins  are  based.  In  addition,  one  will  readily 
gain  the  impression  that  there  is  a  very  wide  diver- 
sity of  interpretations  of  the  facts,  based  largely  up- 
on assumed  analogies  with  conditions  found  in  other 
vertebrates  and  even  invertebrates.     About  the  only 


VARIOUS  KINDS  OF  HUMAN  TWINS  15 

conclusion  which  is  actually  warranted  by  the  facts  is 
that  there  are  two  kinds  of  human  twins,  fraternal  (dizy- 
gotic) and  duplicate  (monozygotic).  There  is  such  a 
diversity  of  conditions,  however,  that  it  is  impossible 
in  some  cases  to  decide  whether  a  given  set  belongs  to 
one  or  the  other  category.  This  situation  is  in  marked 
contrast  with  that  seen  in  the  armadillos  (chap,  ii), 
where  the  intra-uterine  relations  are  practically  uniform 
in  all  pregnancies,  indicating  that  the  peculiar  mechan- 
ism which  brings  about  twinning  is  rigidly  standardized. 
In  human  twins,  however,  twinning  is  by  no  means 
a  single,  fixed  process,  but  is  highly  variable,  evidently 
beginning  earlier  and  being  more  complete  in  some 
cases  than  in  others.  Wilder  is  probably  correct  in 
considering  that  the  various  symmetrical  types  of  double 
monsters  belong  to  the  same  series  as  separate  duplicate 
twins.  I  would  suggest  that  they  are  merely  more  or 
less  incompletely  separated  monozygotic  twins,  in  which 
the  twinning  process  begins  later  than  in  the  separate 
twins  and  then  fails  to  be  fully  carried  out. 

Further  evidence  that  duplicate  twins  are  monozy- 
gotic is  derived  from  a  study  of  identity  between  twins 
and  certain  cases  of  symmetry  reversal  seen  between 
them.  These  matters  will  be  taken  up  in  a  subsequent 
chapter. 

There  is,  as  we  have  seen,  some  embryological 
evidence  derived  from  the  fetal  membranes  of  certain 
twins  that  they  are  monozygotic.  There  are  also 
certain  twins  strikingly  identical.  Unfortunately,  how- 
ever, there  is  no  case  in  which  both  kinds  of  data  have 
been  secured  for  the  same  sets  of  twins.  This  is  the 
information  which,  together  with  the  data  on  the  corpora 


1 6  THE  BIOLOGY  OF  TWINS 

lutea,  would  be  necessary  to  raise  the  phenomenon 
of  monozygotic  twinning  in  man  from  the  realm  of 
probabiHty  to  that  of  demonstrated  fact.  Even  if 
these  data  were  obtained,  we  should  still  be  far  from 
a  real  understanding  of  the  mechanics  of  twinning 
in  man. 

THEORIES   OF   THE   MODE    OF   TWINNING   IN  MAN 

Assuming  that  duplicate  twins  in  man  are  monozy- 
gotic, how  is  twinning  accompHshed  ?  In  the  quotation 
of  Schultze  and  from  Wilder's  comments  we  may  glean 
a  knowledge  of  practically  all  the  various  hypotheses 
on  this  subject.  Some  of  these  are  scarcely  of  sufficient 
importance  to  deserve  notice.  Origin  from  a  double- 
yolked  egg,  for  example,  would  scarcely  be  possible  in 
a  mammal.  Wilder  rightly  excludes  the  possible  origin 
of  twins  from  binucleated  eggs,  because  there  would 
have  to  be  two  sperms,  and  this  would  introduce  a 
degree  of  diversity  that  does  not  occur.  Von  Kolliker's 
suggestion  that  a  single  egg  may  occasionally  produce 
two  embryonic  areas  which  might  or  might  not  develop 
separate  amnions,  depending  on  the  degree  of  juxta- 
position of  the  two  areas,  is  well  worth  consideration. 
It  seems  far  more  probable  than  Wilder's  original  theory 
of  the  complete  separation  of  the  two  blastomeres 
resulting  from  the  first  cleavage  division  of  a  fertilized 
egg.  Wilder  has  recently  abandoned  this  extreme 
''blastotomy"  theory  under  the  influence  of  the  dis- 
coveries of  Newman  and  Patterson  on  the  armadillo, 
and  is  inclined  to  agree  with  these  writers  in  their 
suggestion  that  the  origin  of  human  dupHcate  twins 
probably  resembles  that  of  the  armadillo  quadruplets. 


VARIOUS  KINDS  OF  HUMAN  TWINS  17 

The  theory  that  duplicate  human  twins  arise  by  some 
sort  of  early  fission  process,  probably  initiated  in  the 
ectoderm,  is  rendered  probable  by  the  facts  observed 
in  the  earliest  known  human  embryos,  notably  those 
of  Peters,  of  Frassi,  and  of  Bryce  and  Teacher.  It 
seems  certain  that  the  amnion  in  the  human  embryo 
is  not  a  folding  of  the  somatopleure,  but  arises  as  a 
hollow  in  a  ball  of  ectoderm,  as  in  the  armadillo.  This 
type  of  amnion  formation  would  furnish  the  requisite 
mechanism  for  the  production  of  ectodermic  outgrowths 
from  which,  as  in  the  armadillo  Dasypus,  the  primordia 
of  twins  could  take  their  origin. 

Though  abandoning  the  crude  idea  of  ^'blastotomy" 
origin  of  duplicate  twins,  Wilder  still  chngs  to  the  idea 
that  the  hereditary  differentiation  between  the  twins 
is  to  be  traced  back  to  events  taking  place  during  the 
first  cleavage.  In  this  he  agrees  with  my  own  position 
taken  in  connection  with  armadillo  quadruplets. 

My  opinion  regarding  the  mode  of  origin  of  human 
dupHcate  twins  was  stated  in  the  concluding  paragraph 
of  my  latest  paper  on  "Heredity  and  Symmetry  in 
Armadillo  Quadruplets  "  :^ 

I  am  inclined  to  believe  that  duplicate  human  twins  become 
physiologically  isolated  at  a  considerably  earlier  period  than  do 
armadillo  quadruplets,  and  my  reason  for  this  belief  is  founded 
on  the  fact  that  there  is  so  little  mirror-imaging  in  the  former  and 
so  much  in  the  latter.  It  appears  to  be  a  good  general  rule  that 
the  earlier  the  separation  the  more  complete  is  the  reorganiza- 
tion of  symmetry  relations  in  the  separate  individuals  and  the 
less  residuum  of  the  original  common  symmetry.  Double  mon- 
sters doubtless  begin  to  separate  comparatively  late  in  ontogeny 
and  hence  (sometimes)   show  very  pronounced  mirror-imaging. 

^  H.  H.  Newman,  Biological  Bulletin,  XXX,  No.  2  (1916). 


1 8  THE  BIOLOGY  OF  TWINS 

Since  it  seems  entirely  probable  that  human  duplicate  twins  are 
separated  at  a  much  earlier  period  than  are  armadillo  quad- 
ruplets, it  may  not  be  unreasonable  to  look  for  this  separation  at 
some  period  of  cleavage.  Or  there  may  be  a  division  of  the  inner- 
cell  mass  into  the  primordia  of  the  two  embryos.  The  problem 
of  the  exact  origin  of  duplicate  human  twins  is,  however,  likely 
to  remain  unsolved  for  a  long  time  to  come. 

CONJOINED   TWINS   AND   DOUBLE   MONSTERS 

Double  individuals  have  for  a  long  time  attracted 
the  attention  of  obstetricians  and  embryologists,  and 
there  is  an  extensive  literature  on  the  subject,  with 
little  value,  however,  for  a  book  of  this  sort.  Certain 
of  the  lower-grade  conjoined  twins,  the  Siamese  twins, 
for  instance,  have  stimulated  human  curiosity  to  such 
an  extent  that  they  have  been  exploited  as  freaks  in 
circus  side  shows  and  museums  the  world  over.  The 
late  P.  T.  Barnum  owed  a  considerable  measure  of  his 
early  success  to  his  discovery  and  exhibition  of  the 
Siamese  brethren.  Every  degree  of  junction  between 
twins  has  been  noted,  ranging  from  mere  fusion  of 
parts  of  the  skin  that  can  be  and  have  been  cut  apart 
without  injury  to  extreme  unions  involving  the  head 
and  entire  trunk.  An  abbreviated  classified  list  of 
types  of  composite  monsters,  based  on  Wilder's  ex- 
tensive data,  will  serve  to  show  the  range  of  forms 
included. 

There  are  distinguished  two  main  types:  one  ''in 
which  the  components  or  component  parts  are  equal 
to  and  the  symmetrical  equivalents  of  one  another: 
Diplopagi^'' ;  the  other,  ''unequal  and  asymmetrical 
monsters,  one  component  of  which  is  smaller  than  and 
dependent  upon  the  other:   Autosite  and  Parasite. 


?> 


VARIOUS  KINDS  OF  HUMAN  TWINS  19 

1.  Diplopagi. — These  are  subdivided  into  two  groups 
characterized  as  follows:  (a)  "each  of  the  two  com- 
ponents complete  or  nearly  so";  {h)  "the  two  com- 
ponents equal  to  one  another,  but  each  one  less  than 
an  entire  individual."  In  the  first  group  are  included 
a  considerable  number  of  subtypes  based  on  the  point 
of  juncture,  some  being  joined  at  the  head,  others  at 
the  thorax,  others  at  the  sacrum.  Names  have  been 
given  to  these  types  as  follows:  craniopagi,  thoracopagi, 
and  pygopagi.  They  may  be  back  to  back,  side  by 
side,  or  face  to  face.  In  fact,  the  point  of  juncture  may 
be  such  that  they  are  united  so  as  to  face  at  almost  any 
angle.  Yet  it  must  be  noted  that  they  are  not  joined  in 
any  haphazard  way,  but  are  so  placed  that  they  form 
two  bilaterally  symmetrical  objects.  In  fact,  one  is 
the  mirror-image  of  the  other,  the  plane  of  the  imaginary 
mirror  being  that  in  which  the  junction  exists.  A 
number  of  types  of  cosmohia  or  symmetrical  conjoined 
twins  are  shown  in  Figs,  i  and  2,  and  are  described  in 
the  legends. 

2.  Autosite  and  parasite. — Sometimes  the  parasite 
is  merely  a  head  or  a  head  and  arms  attached  to  the 
autosite  at  or  near  the  epigastrium  or  upper  part  of  the 
abdomen.  At  other  times  the  parasite  consists  of  legs 
and  part  of  the  lower  body,  without  a  head,  attached 
as  in  the  first  case.  Or,  finally,  the  parasite  may  be  a 
supernumerary  head  or  face  attached  to  the  side  or  back 
of  the  head  of  the  autosite. 

Extreme  conditions  are  those  in  which  the  parasite 
is  within  the  autosite.  These  form  tumors  usually  in 
the  body  cavity.  They  range  from  almost  complete 
fetuses  to  mere  masses  of  tissue  sometimes  containing 


^lUT  ^e(e^  -tj  Jtiuib 


J^alC^^i  z-^Sumb 


Fig.  I. — Various  types  of  double  monsters  (from  Wilder).  These 
are  all  strongly  conjoined  and  consist  of  less  than  two  complete  individ- 
uals. Note  the  inequality  in  size  of  the  right-hand  middle  pair,  the 
median,  partially  double  arms  and  legs  in  several  of  the  twins,  and  the 
symmetrical  relations  of  the  two  individuals  in  all  cases. 


VARIOUS  KINDS  OF  HUMAN  TWINS 


21 


teeth  or  hair.  One  strange  case  of  parasite  and  auto- 
site  is  cited  by  BlundelP  and  may  be  worth  describing 
in  detail.     This  is  the  case  of  "a  boy  who  was  literally 


Fig.  2. — The  upper  figure  shows  Renault's  twins  (after  Wilder), 
which  approach  the  condition  of  the  Siamese  twins,  where  both  individ- 
uals are  complete  or  nearly  so.  The  lower  twins  are  typical  Janus 
monsters:  (a)  a  Cyclopian;  (b)  a  case  in  which  what  appears  to  be 
a  single  broad  face  is  really  a  double  face  in  which  the  inner  half  of  each 
component  has  been  suppressed. 

and  without  evasion  with  child,  for  the  fetus  was 
contained  in  a  sac  communicating  with  the  abdomen 
and  was  connected  to  the  side  of  the  cyst  by  a  short 


London  Lancet,  1828-29,  ?•  260, 


22  THE  BIOLOGY  OF  TWINS 

umbilical  cord;  nor  did  the  fetus  make  its  appearance 
until  the  boy  was  eight  or  ten  years  old,  when,  after 
much  enlargement  of  pregnancy  and  subsequent  flooding, 
the  boy  died." 

MODE   OF    ORIGIN    OF   DOUBLE   MONSTERS 

Wilder  in  his  monograph  of  1904  considers  that 
double  monsters  are  simply  united  or  unseparated 
duplicate  twins: 

To  one  who  sees  in  separate  twins  the  result  of  the  total 
separation  of  the  first  two  blastomeres  of  a  developing  ovum  there 
is  but  one  rational  explanation  of  diplopagi,  or  those  composite 
monsters  in  which  the  two  components  are  the  duplicates  of  one 
another  and  symmetrically  united,  namely,  that  here  a  similar 
tendency  to  separation  has  been  left  incomplete,  causing  doubling 
of  those  parts  only  in  which  the  interrelations  have  been  severed. 

Recently  Wilder  has  abandoned  his  idea  that  duplicate 
twins  and  double  monsters  are  derived  by  the  complete 
or  partial  separation  of  the  first  two  blastomeres,  but 
adheres  firmly  to  the  idea  that  each  individual  traces 
back  its  cell  lineage  to  one  of  those  cells. 

Various  other  theories  involving  the  idea  of  fusion, 
as  opposed  to  that  of  incomplete  separation,  have  been 
advanced.  Fischer  as  early  as  1866  supposes  that 
double  monsters  are  the  result  of  an  early  total  fission 
of  the  embryo  followed  by  a  secondary  fusion  of  parts. 
He  says  that  they 

are  invariably  the  product  of  a  single  ovum,  with  a  single  vitellus 
and  vitelline  membrane,  upon  which  a  double  cicatricula,  or  two 
primitive  traces,  are  developed.  The  several  forms  of  double 
malformation,  the  degree  of  duplicity,  the  character  and  extent 
of  the  fusion,  aU  result  from  the  proximity  and  relative  position 


VARIOUS  KINDS  OF  HUMAN  TWINS  23 

of  the  neural  axes  of  the  two  more  or  less  definite  primjl  ive  traces 
developed  on  the  vitelline  membrane  of  a  single  ovum. 

Considering  how  little  was  really  known  of  embry- 
ology at  the  time,  this  idea  of  Fischer  shows  surprising 
insight.  I  am  inclined  to  believe  that  this  explanation 
comes  very  close  to  the  true  one.  It  will  be  remembered 
that  in  the  account  of  intra-uterine  relations  of  duplicate 
twins  a  condition  was  described  in  which  the  twins  were 
not  only  monochorial  but  mon-amniotic  (contained 
within  a  single  amnion) .  This  appears  to  me  to  present 
many  possibilities  for  fusions.  If  we  suppose  that 
twins  arise  by  some  process  of  fission,  in  general  like 
that  in  the  armadillo,  it  is  likely  that  in  some  cases  the 
outgrowths  arise  so  close  together  that  only  a  single 
amnion  is  formed,  and  in  such  cases  the  visceral  parts 
in  particular  would  be  likely  to  fuse  or  remain  unsep- 
arated,  as  in  the  more  pronounced  types  of  diplopagi. 
Even  in  those  cases  that  succeed  in  maintaining  a  dis- 
junction until  the  body  parts  are  completely  separated 
it  would  be  possible,  or  even  highly  probable,  that, 
through  the  crowding  incident  upon  growth  within  one 
amnion,  surface  fusions  more  or  less  extensive  would 
occur.  Wilder  offers  as  an  objection  to  this  and  other 
theories  involving  the  idea  of  secondary  fusion  of 
originally  separate  primordia,  that  it  is  incompatible 
with  the  fact  that  the  two  components  of  a  double 
monster  are  strictly  bilaterally  symmetrical  and  that 
"there  is  no  force  to  oversee  and  adjust  the  two  com- 
ponents in  the  exact  relationship  necessary  for  the 
result."  This  objection  loses  force,  I  believe,  if  the  two 
components  are  viewed  as  outgrowths  lying  in  a  bilateral 
position  on  the  germ  and  held  in  that  position  by  their 


24  THE  BIOLOGY  OF  TWINS 

embryonic  connections.  It  is  hardly  likely  that  two 
outgrowths  could  turn  around  or  reverse  their  positions, 
at  least  not  until  the  umbilical  cords  became  elongated. 
Even  armadillo  quadruplets  which  are  in  separate  amnia 
are  found  in  advanced  stages  lying  in  positions  such 
that,  should  fusions  occur  between  adjacent  individuals, 
they  would  unite  to  form  diplopagi  strictly  bilaterally 
symmetrical  with  reference  to  one  another. 

It  is  highly  probable  that  certain  cases  of  doubling 
in  head  and  posterior  regions  of  human  twins  are  due 
to  disturbances  in  the  process  of  concrescence  of  the 
right-  and  left-hand  component  of  such  embryonic 
primordia  as  the  neural  groove  and  the  ventral  body 
suture.  In  various  vertebrates  all  sorts  of  partial  dou- 
bling may  be  produced  by  experiment.  For  example, 
I  have  worked  with  certain  strains  of  hybrid  fish  in 
which  double-headed  and  double-tailed  fish  have  been 
the  result  of  abnormal  developmental  conditions.  It 
may  well  be  true,  then,  and  probably  is,  that  not  all 
cases  of  doubling  in  human  embryos  are  due  to  the 
same  type  of  cause;  some  may  be  due  to  fission  and 
others  to  fusion.  Where  we  have  so  little  evidence  on 
the  embryological  side,  it  seems  unwise  to  speculate 
further  upon  the  causes  of  twinning  and  doubling  in  man. 

The  only  real  clue  as  to  the  mode  of  twinning  in 
man  comes  from  a  study  of  polyembryonic  development 
in  the  armadillo.  For  various  reasons  it  is  believed 
that  the  process  of  monozygotic  twinning  in  man  is 
essentially  the  same  as  that  of  the  armadillo;  hence 
a  detailed  study  of  the  facts  revealed  by  a  study  of 
the  armadillo  is  the  nearest  approach  possible  today 
to  a  direct  study  of  human  twinning. 

M  r  <itnu  Colkne 


CHAPTER  II 

TWINNING   (POLYEMBRYONY)    IN   THE   ARMADILLO, 

DASYPUS  NOVEMCINCTUS 
HISTORICAL 

Polyembryony^  is  exhibited  by  two  closely  allied 
species  of  armadillo  belonging  to  the  genus  Dasypus^ 
{Tatusia  or  Tatu  of  some  writers),  D.  novemcinctus 
and  D.  hyhridus.  Certain  significant  facts  have  been 
known  about  the  latter  species  for  about  thirty  years. 
H.  von  Jh'ering  in  1885  and  in  1886  published  two  brief 
notes  in  which  he  stated  that  he  had  secured  two  preg- 
nant females  of  this  species,  the  uterus  of  each  of  which 
contained  eight  fetuses,  all  inclosed  within  a  single 
chorion.  All  in  both  sets  were  males.  On  the  basis 
of  this  observation  he  published  in  a  subsequent  note 
the  suggestion  that  all  of  the  embryos  of  each  set  were 
a  product  of  the  splitting  of  a  single  fertihzed  egg  into 
a  number  of  separate  embryonic  primordia.  Although 
the  bearings  of  this  situation  on  important  problems 
of  sex-determination  and  heredity  were  not  appreciated 
by  von  Jhering,  it  must  be  acknowledged  that,  by  a 
flash  of  insight,  he  arrived  at  an  entirely  correct  diagnosis 
of  the  underlying  biological  significance  of  the  observed 
conditions. 

^  Polyembryony  is  a  unique  mode  of  twinning  in  which  plural 
offspring  are  derived  from  a  single  fertilized  egg. 

^  Although  most  writers  who  have  dealt  with  this  species  have 
given  it  the  generic  name  Tatusia,  the  laws  of  priority  favor  the  generic 
name  Das y pus. 

25 


26  THE  BIOLOGY  OF  TWINS 

Further  investigations  concerning  the  mechanics  of 
this  unique  type  of  reproduction  were  forestalled  by  the 
curiously  misleading  investigation  of  Rosner,^  who 
in  1 90 1  published  an  account  of  a  microscopic  study  of 
the  ovaries  of  a  single  specimen  of  D.  novemcinctus 
sent  him  by  von  Jhering.  Rosner  found  that  many 
of  the  ripe  follicles  contained  several  eggs  and  that 
the  two  largest  follicles  each  contained  four  eggs,  a 
number  corresponding  to  that  of  the  fetuses  of  a  litter. 
On  the  basis  of  these  observations  he  decided  that 
von  Jhering's  theory  of  the  origin  of  the  set  of  embryos 
from  a  single  fertilized  egg  was  incorrect,  and  that  the 
four  embryos  typical  for  a  litter  in  D.  novemcinctus  were 
derived  from  a  nest  of  four  eggs  inclosed  in  a  single 
folhcle.  No  attempt  was  made  to  account  for  the 
fact  that  all  in  a  litter  were  of  the  same  sex.  Unfor- 
tunately Rosner  happened  to  examine  a  pair  of  patho- 
logical ovaries,  as  has  been  abundantly  shown  through 
my  own  studies  and  confirmed  by  the  independent 
observations  of  several  other  investigators.  It  is  very 
unusual  to  find  more  than  a  single  egg  in  a  folhcle;  in 
fact,  only  one  pair  of  ovaries  out  of  nearly  thirty  that 
have  been  sectioned  for  my  studies  shows  any  but 
normal  monovic  folhcles.  Since,  as  Rosner  erroneously 
claimed,  the  four  quadruplets  came  from  four  fused  ova, 
the  problem  appeared  to  be  solved.  On  this  account, 
perhaps,  no  special  interest  was  taken  in  the  armadillo 
situation  for  some  time. 

In  the  year  1909  the  question  was  reopened,  when 
almost  simultaneously  there  appeared  preliminary  ac- 
counts  of    the  conditions  in  two  species  of  Dasypus. 

^  M.  A.  Rosner,  Bull.  Acad.  Sci.  de  Cracovie,  1901. 


TWINNING  IN  DASYPUS  NOVEMCINCTUS         27 

Fernandez'^  published  an  account  of  several  embryonic 
stages  of  the  Mulita  {D.  hyhridus)  taken  in  the  Argentine 
Repubhc;  and  Newman  and  Patterson^  published  a 
preliminary  report,  based  upon  some  advanced  embry- 
onic stages,  of  the  conditions  existing  in  D.  novefncinclus 
Texaniis — the  Texas  armadillo.  Since  the  conditions 
in  the  two  species  are  essentially  similar  and  since  much 
more  has  been  published  about  the  latter  species,  the 
main  account  of  polyembryony  in  the  armadillos  will  be 
based  upon  conditions  worked  out  for  the  Texas  species. 

ECOLOGY   AND   HABITS    OF   THE    TEXAS   ARMADILLO, 

Dasypus  novemcinctus  Texanus 

The  average  adult  armadillo  (see  frontispiece)^  is 
an  animal  with  a  body  length  of  about  eighteen  inches, 
with  a  long,  sharp  nose,  mulish  ears,  and  a  pointed 
tapering  tail  nearly  as  long  as  the  head  and  body 
together.  The  most  striking  structural  feature  is  the 
armor,  which  consists  of  a  carapace  composed  of  a 
solid,  scapular  shield  anteriorly,  a  pelvic  shield  poste- 
riorly, and  a  median  banded  region,  consisting  of  nine 
movable  bands  of  armor.  There  is  a  cephahc  shield  on 
top  of  the  head  and  the  tail  is  composed  of  rings  of  armor 
plate  separated  by  armorless  rings  of  soft  skin.     The 

^  M.  Fernandez,  MorpJiolog.  Jahrb.,  Bd.  39  (1909). 

2  H.  H.  Newman  and  J.  T.  Patterson,  Biological  Bulletin,  X\'II, 
No.  3  (1909). 

3  This  illustration,  painted  by  Mr.  Kenji  Toda  from  photographs 
of  living  animals,  is  the  only  really  good  figure  of  an  adult  Dasypus 
of  any  species  that  has  been  pubhshed.  The  usual  figures  are  from 
photographs  of  stuffed  specimens  in  entirely  incorrect  attitudes  and 
in  unnatural  surroundings.  This  picture  is  in  every  respect  an 
adequate  representation  of  this  interesting  animal. 


28  THE  BIOLOGY  OF  TWINS 

head  with  its  long  ears  looks  a  little  like  that  of  a  mule. 
The  feet  are  armed  with  heavy  claws  adapted  for  burrow- 
ing, and  the  legs,  which  are  incompletely  covered  with 
scales  and  hair,  are  comparatively  short.  Many  thou- 
sands of  the  adult  animals  are  slaughtered  annually  for 
their  armatures,  which  are  shaped  into  baskets,  the  tail 
being  arched  over  and  tied  to  the  snout  for  a  basket 
handle.  Armadillo  baskets  are  now  fairly  familiar 
curios  the  world  over.  By  co-operation  with  the  basket 
merchants  it  has  been  possible  to  obtain  an  abundance 
of  material  for  embryological  study  without  in  any  way 
augmenting  the  slaughter  of  this  species,  which  is  going 
on  at  an  alarming  rate. 

The  armadillo  spends  its  life  on  the  defensive  and  its 
equipment  for  defense  consists  of  numerous  structural 
and  functional  adaptations  to  a  very  special  environ- 
ment. The  armor  is  significant  chiefly  as  a  protection 
from  the  thorns  and  spines  of  the  arid  vegetation  in 
which  the  animal  feeds  and  into  which  it  retreats  for 
shelter  from  enemies  and  from  the  tropic  sun.  Doubt- 
less, too,  the  armor  is  of  service  as  a  defense  against 
predacious  enemies,  but,  if  one  may  judge  by  the 
armadillo's  method  of  defense  against  hunting  dogs, 
the  armor  is  less  effective  than  the  claws.  Stories  of 
the  armadillo  rolling  up  into  a  balP  when  attacked 
are  totally  inapplicable  to  this  species,  for  the  animal 
turns  over  on  its  back  and  kicks  viciously  and  effectively 
with  its  powerful  and  heavily  armed  feet.  Although 
an  advocate  of  defensive  armament,  the  armadillo 
believes  in  active  as  well  as  passive  defense. 

^  The  little  armadillo  Tolypeutes  is  the  only  one  that  rolls  up  into 
a  ball. 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        29 

Armadillos  are  pre-eminently  insectivorous,  although 
not  exclusively  so.  Insect-hunting  is  carried  on  at 
night  or  at  dawn  and  dusk.  On  warm  nights  one  may 
hear  the  sniffing  and  grunting  noises  made  by  the 
armadillos  as  they  root  about  among  the  dry  leaves  and 
ground  vegetation  after  the  manner  of  hogs.  In  the 
daytime  they  retire  to  their  burrows,  which  are  dug 
to  a  depth  of  six  or  seven  feet  in  the  dry  soil.  An  en- 
larged chamber  at  the  bottom  is  filled  with  dry  leaves 
into  which  the  animal  burrows  for  warmth  in  the 
winter  and  during  the  cool  spells  of  autumn  and  spring. 
It  is  in  the  burrow  that  the  young  are  born  and  reared. 

Mating  occurs  in  October  and  the  period  of  gestation 
is  between  four  and  five  months.  The  young  are  quite 
advanced  at  birth  and  are  able  to  walk  about  within 
the  first  few  hours. 

This  incomplete  account^  of  the  natural  history  of 
the  armadillo  is  given  here  merely  to  enable  the  reader 
to  gain  a  slight  acquaintance  with  the  species  that  has 
furnished  the  embryonic  material  forming  the  central 
subject-matter  of  the  present  volume.  In  this  place  it 
may  not  be  inappropriate  to  extend  to  the  modest  and 
retiring  armadillo  an  acknowledgment  of  our  indebted- 
ness for  much  valuable  data  that  no  other  animal  could 
have  supplied. 

Our  studies  of  the  development  of  the  armadillo 
cover  the  whole  range  of  stages  from  ovogenesis  to 
birth,  with  but  one  gap  which,  it  is  hoped,  the  near 
future  will  see  filled  in.     It  has  been  impossible  so  far 

*  A  somewhat  more  adequate  account  of  the  natural  history  of  this 
species  may  be  found  in  the  following  paper:  H.  H.  Newman,  American 
Naturalist,  XL VII  (19 13). 


30 


THE  BIOLOGY  OF  TWINS 


to  secure  the  earliest  cleavage  stages  of  the  normally 
developing  egg,  but  a  study  of  parthenogenetic  cleavage,^ 
as  it  occurs  in  atretic  follicles,  has  been  made;  these 
data  undoubtedly  foreshadow  some  of  the  most  funda- 
mental events  of  normal  cleavage.  Until  a  study  of 
normal  cleavage  is  forthcoming  the  facts  of  partheno- 
genetic cleavage  may  be  accepted  as  a  temporary 
substitute. 


Fig.  3. — Uterus,  ovaries,  etc.,  of  adult  Dasypus  novemcindus  (arma- 
dillo), showing  simple  squarish  uterus  with  sharp  fundus  end  (fu), 
cervix  (c),  Fallopian  tube  (ft),  ovaries  (0),  only  one  of  which,  the  left, 
has  a  corpus  luteum  (cl).     (From  Newman  and  Patterson.) 


THE  FEMALE   GENITALIA 

The  uterus  of  the  armadillo  is  simple  and  quite 
Hke  that  of  man  and  of  other  primates  which  produce 
but  one  offspring  at  a  birth.  Viewed  from  the  dorsal 
aspect,  the  non-pregnant  uterus  appears  to  be  broadly 
kite-shaped  (Fig.  3),  with  the  posterior  angle  blending 
into  the  vagina  and  with  the  right  and  left  corners 
connecting  with  the  Fallopian  tubes.    The  free  or  fundus 

»H.  H.  Newman,  Biological  Bulletin,  XXV,  No.  i  (1913)- 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        31 

end  (/w)  of  the  uterus  runs  to  a  point.     Internally  two 
grooves  in  the  mucosa  intersect  at  the  very  apex  of  the 


_."%::.  ..... 

Fig.  4. — The  small  spherical  body  near  the  center  of  the  cross- 
shaped  area  is  an  armadillo  egg  beginning  to  adhere  to  the  uterine 
membranes  in  the  fundus  of  the  uterus.  The  wrinkled,  lighter  area 
surrounding  the  specialized  attachment  area  is  the  general  uterine 
mucous  membrane.  The  lateral  arms  of  the  cross-shaped  area  are 
grooves  communicating  with  the  right  and  left  oviducts  or  Fallopian 
tubes.     (From  Patterson.) 

fundus,  and  it  is  at  this  point,  the  center'  of  a  cross- 
shaped  smooth  area,  that  the  ^^g  attaches  itself 
when    the    placenta tion   occurs    (Fig.   4).     The   whole 

I  If  the  egg  has  come  from  the  right  ovary,  it  will  become  attached, 
as  in  the  figure,  somewhat  to  the  right  of  center;  if  from  the  left  ovary, 
to  the  left  of  center. 


32  THE  BIOLOGY  OF  TWINS 

mechanism  is  adapted  to  accommodate  a  single  develop- 
ing egg. 

There  is  nothing  suggestive  of  polyembryony  about 
the  ovaries  or  oviducts.  Each  ovary,  when  no  corpus 
luteum  is  present,  is  kidney-shaped  and  about  the 
size  of  a  small  bean;  in  virgin  females  the  two  are  of 
the  same  size.  In  every  pregnant  female,  however, 
one  of  the  ovaries  is  several  times  as  large  as  the  other, 
owing  to  the  presence  of  an  enormous  corpus  luteum 
in  that  ovary  which  has  produced  the  fertilized  ovum. 
As  in  many  mammals,  the  corpus  luteum  of  pregnancy 
is  an  extremely  conspicuous  object,  especially  in  its 
earlier  phases,  and  its  presence  is  unmistakable.  One 
of  the  earliest  and  most  important  discoveries  in  the 
armadillo  investigation  was  that  there  is  never  more 
than  one  true  corpus  luteum  in  the  ovaries  of  a  pregnant 
female.  That  the  number  of  corpora  lutea  is  a  safe 
criterion  of  the  number  of  eggs  involved  in  a  pregnancy 
is  generally  recognized  by  embryologists,  and  we  may 
feel  safe  in  applying  this  test  in  cases  such  as  those 
offered  by  human  and  by  ungulate  twins  where  the 
early  embryonic  history  is  unknown.  The  evidence 
of  the  corpus  luteum  that,  in  the  armadillo,  only  one 
egg  is  produced  and  fertilized  at  a  pregnancy  is  supported 
by  a  study  of  ovogenesis,  maturation,  and  fertilization. 

OVOGENESIS 

The  process  of  ovogenesis^  (development  of  eggs) 
in  the  armadillo  presents  in  the  earlier  stages  at  least 
nothing  unusual,  but  is,  on  the  contrary,  quite  typical 
for  mammals  in  general.     Each  of  the  young  ovocytes 

^  H.  H.  Newman,  Biological  Bulletin,  XXIII  (191 2). 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        33 


gv 


(immature  eggs)  develops  its  own  separate  follicle,  and 

the  development  of  both  ovocyte  and  follicle  is  much 

like  that  of  the  mouse  or  the  cat.     In  only  a  very  few 

instances  has  a  foUicle  with  two  or  more  ovocytes  been 

observed,    and   many   ovaries    totally   lack   double    or 

multiple  follicles.     The  full-grown  ovocyte,  which  has 

a  diameter  of  about  12  micra,  is  a  little  smaller  than 

that  of  the  cat  and  a  Httle  larger  than  that  of  man 

or   those  of  rodents. 

Prior   to   maturation 

the  definitive  ovocyte 

of  the  first  order  lies 

in  the  discus  pro- 

ligerus,  a  mass  of 

follicular  cells,  which 

adheres   to   one    side 

of  the  large  follicular 

cavity. 

A  cytological  ex- 
amination of  the 
full-grown  ovocyte 
(Fig.  5)  shows  that 
there  exists  a  pro- 
nounced cellular 
polarity.  The  germinal  vesicle  is  flattened  against  the 
zona  pellucida  presumably  at  the  animal  pole.  A 
comparatively  homogeneous  zone  of  darkly  staining 
protoplasm  which  is  thicker  at  the  pole  occupied  by 
the  germinal  vesicle  surrounds  a  sphere  of  coarsely 
vacuolated  material,  which  must  be  identilied  as  the 
deutoplasmic  zone  or  yolk  mass,  in  which  are  scattered 
bits   of   yolk  material   suspended  in   a  fluid   medium. 


Fig.  5. — Full-grown  ovocyte  (un- 
maturated  egg)  of  the  armadillo,  showing 
the  mass  of  thin  yolk  in  the  center  {d  s) 
and  the  peripheral  formative  zone  of 
protoplasm  (/s),  the  nucleus  or  germinal 
vesicle  {g  v)  at  the  animal  pole,  and  the 
zona  pelucida  or  egg  shell  {z  p). 


34 


THE  BIOLOGY  OF  TWINS 


dz 


f'   - 


If  the  site  of  the  germinal  vesicle  is  that  of  the  animal 
pole,  the  vegetative  pole  is  that  which  is  most  nearly 
in  contact  with  the  yolk  mass/ 

During  maturation  an  extremely  radical  change  in 
polarity   and   general   organization   takes   place.     The 

comparatively  homo- 
geneous formative 
zone  of  protoplasm 
d^  moves  to  one  pole  of 
the  ^gg  and  forms  a 
cap  with  a  crescentic 
cross-section,  thick 
in  the  middle  and 
thinned  out  at  the 
edges  (Fig.  6).  The 
originally  central 
yolk  mass  moves  to 
the  pole  of  the  egg 
opposite  to  that  occu- 
pied by  the  formative 
cap,  and  comes  to  lie 
in  contact  with  the 
zona  pellucida  for 
about  one-third  of  the  periphery  of  the  egg.  The  germi- 
nal vesicle  during  this  reorganization  process  enters  the 
stage  of  a  first  polar  spindle,  which,  peculiarly  enough, 
lies  tangentially  to  the  periphery  of  the  ovocyte  and  very 
close  to  the  boundary  between  the  formative  and  deu- 

^  This  central  position  of  the  yolk  and  the  peripheral  position  of 
the  formative  protoplasm  are  strikingly  like  those  described  by  Hill 
(19 id)  for  the  marsupial  Dasyurus  and  may  therefore  be  interpreted 
as  a  primitive  character. 


Fig.  6. — Maturating  egg  of  the  arma- 
dillo, showing  total  rearrangement  of 
materials.  The  yolk  {d  z)  with  yolk 
granules  {d  g)  occupies  the  animal  pole, 
and  the  formative  protoplasm  (/  s)  occu- 
pies the  opposite  pole  in  the  form  of  a 
cap.  The  nucleus  {p  s)  is  dividing  to 
form  the  first  polar  body  and  lies  tan- 
gentially to  the  periphery. 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        35 

toplasmic  zones.  This  pronounced  shiftinjij  al^out  of 
materials  within  the  maturing  ovocyte  must  take  place 
in  a  very  brief  space  of  time,  for  it  has  not  been  possible 
to  find  in  a  very  large  number  of  specimens  examined  any 
stages  transitional  between  that  before  the  reorganiza- 
tion begins  (Fig.  5)  and  that  after  its  completion  (Fig.  6). 
In  attempting  to  interpret  the  significance  of  this 
remarkable  cellular  cataclysm  it  is  interesting  to  note 
that  HilP  finds  exactly  the  same  situation  in  the  marsu- 
pial Dasyurus,  which  he  interprets  as  a  complete  reversal 
of  polarity.  According  to  this  writer  the  formative 
protoplasm  now  occupies  the  vegetative  pole  while  the 
yolk  mass  occupies  the  animal  pole.  The  position  of 
the  maturation  spindle  is  in  the  formative  zone,  but 
as  near  the  animal  pole  as  possible.  Whatever  may  be 
the  morphological  interpretation  of  the  changes  in 
cellular  organization  that  take  place  during  maturation, 
there  can  be  little  doubt  as  to  the  physiological  sig- 
nificance of  the  shifts  of  material.  If  we  may  judge 
by  analogy  with  Dasyurus  and  by  a  study  of  partheno- 
genetic  cleavage  in  atretic  follicles  of  the  armadillo, 
the  shift  of  the  deutoplasm  from  the  center  to  the 
periphery  of  the  ovocyte  is  the  first  step  in  the  process 
of  deutoplasmic  extrusion.  This  mass  of  thin  degenerate 
yolk  is  of  no  value  to  the  egg  and  must  be  voided  before 
cleavage  can  begin.  The  process  of  voidancc  appears 
to  be  one  involving  rupture  of  the  vitelline  membrane 
and  abstriction  of  the  yolk,  followed  by  a  subsequent 
rounding  up  of  the  formative  materials  to  form  an  egg 
that  is  much  smaller  than  the  original  ovocyte  and 
is  presumably  rejuvenated  by  the  loss  of  its  deutoplasm. 

^  J.  P.  Hill,  Quarterly  Journal  oj  Microscopical  Science,  LVI  (19 10). 


36  THE  BIOLOGY  OF  TWINS 

Prior  to  the  complete  abstriction  of  the  yolk  mass 
the  egg  completes  its  nuclear  maturation.  Studies 
of  the  chromosomes  show  that  there  are  thirty-two 
in  the  ovocyte  of  the  first  order,  and  that  after  typical 
tetrad  formation  the  number  is  reduced  to  sixteen  in 
the  first  polar  body  and  to  sixteen  in  the  ovocyte  of 
the  second  order.  The  second  maturation  division  is 
equational  and  produces  a  second  polar  body.  It  is 
very  rare,  however,  to  find  a  second  polar  body  formed 
before  the  process  of  ovulation.  This  process  probably 
occurs  while  the  egg  is  in  the  oviduct  just  before  fertihza- 
tion.  After  a  long  search  for  tubal  ova,  I  was  fortunate 
enough  to  find  one  fertilized  egg,  apparently  quite 
normal  in  every  respect,  in  a  part  of  the  oviduct  not 
far  from  the  fimbriated  funnel.  This  egg  showed  just 
two  polar  bodies  and  the  male  and  female  pronuclei 
lying  close  together  in  the  formative  protoplasm  (Fig.  7) . 
The  deutoplasm  had  not  yet  been  extruded.  From  this 
we  might  infer  that  yolk  elimination  occurs  as  an 
accompaniment  of  the  first  cleavage  division,  as  in 
Dasyurus.  A  study  of  parthenogenetic  cleavage^  shows 
numerous  stages  in  which  the  small  yolkless  egg  lies 
within  the  zona  pellucida  surrounded  by  fragments  of 
yolk.  Later  stages  show  very  pretty  cleavage  spindles 
in  the  egg;  clean-cut  four-cell  and  somewhat  doubtful 
eight-cell  stages  have  been  found.  This  is  apparently 
as  far  as  parthenogenetic  development  goes,  so  we 
must  await  the  discovery  of  the  events  in  normal 
cleavage  before  we  can  know  with  certainty  what  form 
of  cleavage  we  have  in  the  armadillo:  whether  it  is 
regular,  as   in   Dasyurus,  or   indeterminate,  as  in  the 

^  H.  H.  Newman,  Biological  Bulletin,  XXV  (1913). 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        37 

majority  of  eutherian  mammals.  I  am  strongly  inclined 
to  predict  that  when  the  cleavage  is  made  known  it  will 
prove  to  be  much  Hke  that  of  Dasyurus.  This  prediction 
rests  on  two  circumstances:  first,  that  the  history  of 
the  ova  of  Dasyurus  and  that  of  Dasypus  are  alike  as 
far  as  we  can  trace  them,  and  that  the  latter  is  unlike 


Fig.  7. — A  fertilized  armadillo  egg  with  two  polar  bodies  and  the 
male  and  female  pronuclei  side  by  side. 

that  of  other  species  of  EutJieria;  second,  that  the 
arrangements  of  embryos  in  pairs  and  the  mirror- 
imaging  effects  that  are  so  striking  a  feature  of  the 
quadruplets  are  much  more  in  accord  with  a  type  of 
cleavage  in  which  the  blastomeres  retain  a  regular  and 
definite  position  than  with  one  in  which  the  cleavage 
cells  shift  about. 


38  THE  BIOLOGY  OF  TWINS 

In  general,  we  may  say  in  concluding  this  account 
of  the  history  of  the  egg  up  to  the  beginning  of  cleavage 
that  there  is  nothing  in  any  way  suggestive  of  poly- 
embryony  about  any  phase  of  the  process.  The  eggs 
are  ovulated  singly,  have  small  polar  bodies,  are  ferti- 
lized by  but  one  sperm,  and  begin  cleavage  in  normal 
fashion,  if  we  may  judge  by  the  data  derived  from  a 
study  of  parthenogenetic  cleavage. 

EARLY  EMBRYONIC   DEVELOPMENT 

Fernandez^  was  the  first  to  describe  for  any  species 
of  armadillo  embryos  in  which  no  beginnings  of  poly- 
embryonic  fission  had  taken  place.  He  studied  several 
early  embryos  of  Dasypus  hybridus,  and  although  his 
material  was  in  rather  poor  condition,  it  was  clear  that 
the  blastodermic  vesicle  was  a  single  individual  in  the 
stage  examined  and  not  unlike  similar  stages  in  some 
of  the  rodents.  In  the  autumn  of  1909  Newman 
and  Patterson^  obtained  a  number  of  stages  of  Dasypus 
novemcinctus  covering  the  period  from  the  primitive 
streak  on  to  birth.  In  the  following  season  a  number 
of  earlier  stages  were  collected,  some  of  the  youngest 
of  which  were  lost  in  fixation.  At  the  end  of  this 
season  these  two  collaborators  divided  forces  on  account 
of  the  removal  of  the  senior  author  from  Texas  to 
Chicago.  In  the  resultant  division  of  the  work  the 
completion  of  the  study  of  embryonic  development 
was  assigned  to  Patterson  and  the  genetic  problems 
to  Newman.     It  took  Patterson  two  more  seasons  to 

^  Fernandez,  loc.  cit. 

2  H.  H.  Newman  and  J.  T.  Patterson,  Journal  of  Morphology, 
XXI  (1910). 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        39 

obtain  the  late  cleavage  and  early  embryonic  stages, 
and  to  his  paper'  of  19 13  we  are  indebted  for  a 
large  part  of  the  following  account  of  this  period  of 
development. 

The  earliest  stages  found  were  collected  on  October 
15;  these  consisted  of  two  eggs  in  the  Fallopian  tubes, 
and  several  eggs  floating  freely  in  the  uterine  cavity. 
In  no  case  was  more  than  one  egg  found  in  the  uterus 
or  tubes  of  one  female.  From  the  fact  that  the  Fallopian- 
tube  eggs  and  all  those  found  free  in  the  uterus  were  in 
almost  precisely  the  same  embryonic  stage,  and  from 
the  additional  fact  that  nearly  every  large  female 
examined  as  late  as  three  weeks  after  the  earliest  date 
mentioned  had  an  egg  in  practically  the  same  stage  of 
development,  it  must  be  concluded  that  there  is  a 
period  of  quiescence  of  about  three  weeks,  during  which 
the  egg  either  remains  at  a  standstill  or  else  develops 
so  slowly  as  to  make  no  perceptible  progress.  Although 
Patterson  draws  no  conclusion  as  to  the  effects  on 
development  of  this  period  of  quiescence,  it  is  my  behef 
that  this  period  holds  the  clue  to  the  physiological 
explanation  of  polyembryony.  Consideration  of  this 
point  is  deferred,  however,  until  the  general  discussion 
of  the  underlying  causes  of  twinning. 

The  following  account  of  the  embryology  of  Dasypus 
novemcinctus  I  have  built  up  for  the  convenience  of  the 
reader  around  a  number  of  carefully  constructed  ligures; 
although  slightly  diagrammatic,  they  are  accurate  in 
all  essential  points,  and  are  certainly  more  intelligible 
than  microphotographs,  or  exact  copies  of  preserved 
and  sectioned  material.     I  have  found  that  zoologists 

^  J.  T.  Patterson,  Journal  of  Morphology,  XXIV  (1913). 


40  THE  BIOLOGY  OF  TWINS 

in  general  have  but  little  understood  the  essential 
features  of  polyembryonic  development  in  this  species. 
In  the  hope  that  this  interesting  piece  of  embryology 
may  be  made  quite  definitely  intelligible  to  a  general 
biological  audience,  I  have  had  it  re-illustrated  by 
Mr.  Toda.  The  first  six  figures  (stages  I-VI)  are 
adapted  from  Patterson's  photographs  and  figures;  the 
remaining  stages  represent  material  to  which  I  have  had 
personal  access. 

Stage  /.  The  earliest  embryos  (Fig.  8). — The  young- 
est egg  that  has  been  found  is  in  a  rather  late  cleavage 
stage,  in  which  the  embryonic  cells,  eleven  in  number, 
form  a  small  knot  or  inner-cell  mass  {icm)  attached 
to  the  inner  Surface  of  the  large,  hollow  sphere  of  non- 
embryonic  cells,  the  trophoblast  {tr).  The  trophoblast, 
as  the  name  implies,  has  a  purely  nutritive  function  and 
serves  later  to  attach  the  vesicle  to  the  walls  of  the 
uterus.  Of  the  eleven  cells  seen  in  the  earliest  egg,  six 
differ  from  the  others  in  having  larger  nuclei.  These 
larger  cells  are  destined  to  form  the  embryonic  ectoderm. 
The  other  cells  form  the  endoderm.  Such  an  egg  is  in 
no  way  essentially  different  from  any  eutherian  (higher 
mammalian)  egg,  and  is  unquestionably  at  this  time 
without  any  visible  indications  of  a  prospective  division 
into  four  embryos. 

Stage  II.  The  beginnings  of  gastrulation  (Fig.  9) . — 
The  cells  of  the  inner-cell  mass  have  multiphed  and 
have  spread  out  into  a  flat  disk  of  one  or  two  layers 
in  thickness.  The  distinction  between  ectoderm  (ec) 
and  endoderm  {en)  is  now  evident  in  that  the  less  numer- 
ous endoderm  cells  are  more  deeply  stained  than  the 
ectoderm  cells.     The  endoderm  cells  are  also  beginning 


^     Fig.  8 


Figs.  8,  9. — Sectional  view  of  two  earliest  embryos  of  the  armadillo. 
The  two  are  shown  overlapping,  to  save  space.  (For  description  see 
text,  stages  I  and  II.)  The  trophoblast  (/r)  and  inner-cell  mass  {ic  m), 
ectoderm  {ec),  and  endoderm  {en)  are  shown.  The  point  A''  is  the 
apical  pole  of  the  egg.     (Modified  from  Patterson.) 


Fig.  9 


42  THE  BIOLOGY  OF  TWINS 

to  leave  the  part  of  the  embryonic  disk  that  is  in  contact 
with  the  trophoblast  and  to  migrate  downward  to  a 
position  beneath  the  ectodermal  mass. 

Stage  III.  Gastrulation  completed  (Fig.  lo). — The 
endoderm  cells  have  now  migrated  away  from  the 
trophoderm  and  form  a  complete  layer  of  somewhat 
flattened,  deeply  staining  cells  that  lie  in  close  contact 
with  the  compact  mass  of  ectoderm  cells  {ec)  on  the 
side  away  from  the  trophoblast.^  Very  little  change 
occurs  in  the  trophoblast  during  the  first  three  stages. 
Eggs  of  about  the  stage  shown  in  Fig.  8  are  found 
lightly  attached  to  the  uterine  wall  near  the  middle  of 
the  cross-shaped  area  shown  in  Fig.  4. 

Stage  IV.  Embryonic  germ-layer  inversion  (Fig. 
11). — This  stage  is  one  of  the  most  significant  in  the 
entire  history  in  that  it  shows  a  curious  inversion^  of 
the  normal  relations  of  ectoderm  and  endoderm.  We 
expect  ectoderm  to  be  outside  and  endoderm  to  be  inside, 
but  in  the  armadillo  a  sort  of  inversion  occurs  that 
results  in  the  ectoderm  getting  inside  the  endoderm. 
Although  this  process  is  not  necessarily  followed  by 
twinning,  it  at  least  appears  to  offer  a  highly  favorable 
opportunity  for  this  type  of  embryonic  doubling. 

Soon  after  the  completion  of  gastrulation  the  some- 
what flattened  mass  of  ectoderm  cells  begins  to  round 

^  This  method  of  gastrulation  is  very  strikingly  like  that  described 
by  Hill  for  the  marsupial  cat  Dasyurus,  which  is  of  interest  when  we  recall 
that  the  remarkable  changes  in  the  ovocyte  in  this  species  are  also  like 
those  of  our  armadillo.  One  may  be  fairly  certain  that  the  early  cleavage 
stages  of  the  two  species  will  prove  to  be  similar. 

*  A  very  similar  type  of  germ-layer  inversion  has  been  described 
for  several  species  of  rodent.  The  work  of  Mellisinos  on  the  mouse  is 
especially  interesting  in  this  connection  (Arch.  mikr.  Anat.  und  Entw., 
Bd.  70)  (1907). 


Fig,  io 


Fig.  II 


Figs,  io,  ii. — Two  eggs  of  the  armadillo,  drawn  as  before,  partly 
overlapping.  The  upper  egg  (stage  III)  shows  the  ectoderm  (ec)  round- 
ing up  into  a  ball  and  the  endoderm  (en)  in  the  form  of  a  continuous 
layer  beneath  the  ectoderm.  The  apical  pole  is  at  -Y.  The  lower  egg 
(stage  IV)  shows  the  ectoderm  (ec)  rolled  into  a  ball  and  nearly  sur- 
rounded with  endoderm  (en).  Trophoblast  has  been  thickened  into 
Trager  (Tra)  at  the  upper  pole  and  the  remainder  is  called  diplotropho- 
blast  (dtr);   extra-embryonic  cavity  (exc).     (Modified  from  Patterson.) 


44  THE  BIOLOGY  OF  TWINS 

up  and  the  process  of  rounding  up  is  unquestionably 
a  sort  of  invagination  of  the  middle  or  apical  portions,  so 
that  the  free  edges  unite  above  and  the  whole  mass 
becomes  essentially  a  hollow  ball.  In  Fig.  ii  the  ball 
appears  to  be  only  partially  hollow,  but  the  ectodermic 
mass  is  morphologically  a  vesicle  with  a  central  cavity, 
which  is  the  primitive  amniotic  cavity.  This  invagi- 
nation of  the  ectoderm  involves  a  very  fundamental 
reversal  of  the  primary  axis  of  the  embryonic 
materials,  for  the  head  end'  of  the  ectodermic  mass  is 
now  directed  away  from  the  original  animal  pole  of 
the  egg. 

The  ectoderm  is  the  active  agent  in  this  process  of 
germ-layer  inversion,  and  the  endoderm  plays  the 
merely  passive  role  of  maintaining  its  contact  with 
the  ectoderm.  The  result  is  that  it  comes  almost 
completely  to  surround  the  ectodermic  vesicle.  As  a 
sequel  to  the  inversion  process  the  ectoderm  becomes 
totally  separated  from  contact  with  the  trophoblast, 
and  a  cavity  arises  between  the  latter  and  the  embryonic 
tissues,  which  is  the  beginning  of  the  extra-embryonc 
cavity  {ex  c) . 

That  part  of  the  trophoblast  which  has  adhered 
to  the  uterine  mucosa  has  at  this  period  entered  upon 
a  process  of  rapid  cell  proliferation  preparatory  to 
invading  the  maternal  tissues.  At  this  time  only  short 
protrusions  have  been  formed  that  serve  as  mechanical 
aids  to  adhesion.  This  specialized  region  of  the  tropho- 
blast is   destined   to   form   the  primitive  placenta   or 

^  It  may  be  noted  here  that  in  later  stages  (Figs.  15,  16,  and  17)  the 
heads  of  all  embryos  are  directed  away  from  the  original  apical  end 
or  animal  pole  of  the  egg,  which  is  the  attached  end. 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        45 

Trager,  while  the  thin-walled  part  of  the  trophoblast 
is  known  as  the  diplo trophoblast  (dtr). 

TrCi 
trpi 


Fig,  12, — Au  armadillo  egg  (stage  V)  attached  by  the  Trager  to 
uterus,  and  shown  as  if  torn  away  at  Tra.  The  ectoderm  {ec)  is  a 
hollow  vesicle.  The  apical  pole  is  at  X.  Endoderm  {en)  joins  diplo- 
trophoblast  {dtr)  in  a  ring  {r).  The  trophoderm  plate  {tr  pi)  lies  within 
the  Trager  collar  {Tra);  ectodermal  layer  of  amnion  {cc  am)',  amniotic 
cavity  {ante);  mesoderm  {ms);  much  enlarged  extra-embryonic  cavity 
{exc).     (Modified  from  Patterson.) 

Stage  V.     The  period  of  rapid  growth  and  the  estab- 
lishment 0/  bilaterality   (Fig.    12). — During   the   earlier 


46  THE  BIOLOGY  OF  TWINS 

stages  very  little  increase  in  the  actual  mass  of  tissue 
has  taken  place.  The  egg  has  merely  increased  in 
diameter  owing  to  the  accumulation  of  fluid  in  the 
trophoblast  cavity. 

Simultaneously  with  the  development  of  the  Trager 
and  its  invasion  of  the  uterine  mucosa  there  begins 
a  period  of  rapid  cellular  proliferation  and  consequent 
tissue  growth,  evidences  of  which  have  already  been 
noted  in  Fig.  ii.  At  the  stage  shown  in  Fig.  12  the 
Trager  has  deeply  invaded  the  maternal  mucosa  by  a 
process  analogous  to  that  observed  in  invading  tumors. 
It  has  been  deemed  advisable  to  omit  from  the  drawings 
that  part  of  the  Trager  that  has  penetrated  the  maternal 
tissues  and  to  show  by  broken  cellular  contours  the 
points  (tr)  where  the  vesicle  has  been  severed  from  its 
nutritive  connection  with  the  mother.  A  vesicle  like  that 
shown  in  Fig.  12  is  more  than  twice  as  great  in  diameter 
as  that  shown  in  Fig.  1 1 ,  and  the  increase  in  size  is  due 
in  part  to  the  marked  enlargement  of  the  extra-embryonic 
cavity,  in  part  to  the  expansion  of  the  cavity  of  the 
ectodermic  vesicle,  which  is  now  a  true  amniotic  cavity 
(am  c) .  The  subspherical  ectodermic  vesicle  has  thinned 
out  on  the  side  toward  the  extra-embryonic  cavity  to 
form  the  ectodermal  component  of  the  amnion  {am). 
The  embryonic  ectoderm  is  now  a  vesicular  mass  of  cells 
somewhat  elongated  in  the  bilateral  axis  of  the  uterus  and 
with  anterior  or  apical  end  at  X.  The  embryo  is  really 
a  gastrula  turned  inside  out,  and  hence  the  process 
deserves  the  name  ''germ  layer  inversion."  If  the 
amnion  were  cut  and  the  germ-layers  were  reinverted, 
we  should  get  a  normal  gastrula  with  the  apical  point 
up  and  the  basal  parts  down.     It  should  be  emphasized 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        47 

that  the  embryo  though  inside  out  is  clearly  polarized 
and  bilateral  and  that  it  is  still  one  embryo.  A  further 
evidence  of  bilaterality  is  seen  in  the  mesoderm  {ms) 

Tra 


Fig.  13. — An  armadillo  egg  showing  first  division  into  a  double 
individual.  The  two  first  embryos  (II  and  IV)  are  shown  as  right  and 
left  outgrowths  of  the  ectodermic  vesicle.  (For  details  see  description 
under  stage  VI.     Lettering  as  in  Fig.  12,)     (Modified  from  Patterson.) 

which  is  proliferating  at  two  bilateral  points  where  the 
ectoderm  and  the  endoderm  part  company. 

Stage  VI.     The  first  step  in  twinning — tlie  primary 
embryos  (Fig.  13). — The  ectodermic  vesicle,  which  was 


48  THE  BIOLOGY  OF  TWINS 

originally  situated  at  or  near  the  animal  pole  of  the 
egg,  has  progressively  retreated  from  this  pole  and  now 
lies  with  its  apical  part  (head  end)  almost  at  the  oppo- 
site (vegetative)  pole.  The  formerly  voluminous  cavity, 
shown  in  stages  I-IV,  which  we  have  called  the  tropho- 
blast  cavity,  has  gradually  diminished  in  relative  and 
absolute  volume  (stages  V  and  VI)  until  it  is  merely 
a  fiat  crevice  (ys)  between  the  endoderm  and  the 
trophoblast.  At  this  stage  the  free  edges  of  the  endo- 
derm have  fused  with  the  trophoblast  at  r,  and  that 
portion  of  the  trophoblast  distal  to  the  ring  of  fusion 
(the  diplotrophoblast)  has  thinned  out  preparatory  to  a 
subsequent  total  disappearance,  as  in  stage  VII.  The 
retreat  of  the  ectodermic  vesicle  from  pole  to  pole  and 
the  crowding  out  of  the  original  trophoblast  cavity 
appear  to  be  due  to  the  pressure  of  the  rapidly  enlar- 
ging extra-embryonic  cavity  (ex  c),  which  is  now  lined 
internally  with  a  complete  vesicle  of  mesoderm  (ms). 
That  part  of  the  mesoderm  next  to  the  ectodermal 
amnion  completes  the  amnion  proper.  No  embryonic 
mesoderm  is  as  yet  formed. 

The  ectodermic  vesicle  is  seen  to  be  flattened  against 
the  endoderm,  and  two  hollow  evaginations  are  shown 
at  right  and  left  sides;  these  are  the  primordia  of  the 
primary  embryos  (II  and  IV).  These  outgrowths  con- 
stitute twin  embryonic  areas  with  the  apex  or  head 
end  of  each  pointing  toward  the  apex  of  the  ectodermic 
vesicle  (X),  and  with  the  posterior  or  growing  end  of 
each  pointing  the  one  toward  the  right  and  the  other 
toward  the  left  side  of  the  uterus.  It  seems  quite 
evident  that  the  bilaterahty  of  this  twin  embryonic 
vesicle  has  been  secondarily  imposed  upon  it  by  the 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        49 

bilaterality  of  the  uterus,  for  we  can  see  no  other  way 
of  explaining  the  coincidence  that  exists  between  the 
uterine  and  embryonic  axes.  No  important  change 
has  occurred  in  the  Trager  ring  except  that  further 
invasion  of  the  uterine  mucosa  has  continued.  In 
some  eggs  the  disk  of  the  trophoblast  {tr  d)  lying  within 
the  Trager  ring  appears  to  be  quite  free  from  the 
uterine  mucosa  and  to  form  the  boundary  of  a  more  or 
less  extensive  fluid-filled  cavity,  which  had  been  called 
by  Fernandez  the  Trager  cavity.  This  cavity  is 
evidently  of  little  morphological  significance  and  may 
be  ignored  in  subsequent  stages. 

Stage  VII.  The  origin  of  quadruplets.  Secondary 
embryos  formed  (Fig.  14). — It  is  at  the  stage  shown  in 
Fig.  14  that  the  second  step  in  twinning  occurs,  but 
the  figure,  because  it  is  a  bilateral  sectional  view  of  the 
egg,  fails  to  show  the  secondary  embryos.  The  primary 
embryos  II  and  IV  lie  respectively  to  the  right  and  to 
the  left  of  the  egg,  while  a  shorter  secondary  embryonic 
outgrowth  appears  to  the  left  side  of  each  primary 
embryo,  so  that  the  two  secondary  embryos  lie  with 
their  axes  pointed  one  toward  the  dorsal  and  the  other 
toward  the  ventral  side  of  the  uterus.  The  embryo  on 
the  dorsal  side  is  called  III  and  is  said  to  be  the  second- 
ary embryo  paired  with  the  primary  embryo  IV,  while 
the  ventral  secondary  embryo  is  called  I  and  is  similarly 
related  to  the  primary  embryo  II.  The  outline  sketch 
(Fig.  15)  shows  the  axes  of  the  four  embryos,  seen  from 
the  distal  end  of  a  vesicle  like  that  shown  in  Fig.  14. 
Note  that  there  are  evidences  of  tertiary  outgrowths 
between  I  and  IV  and  between  II  and  III.  In  Dasypus 
hybridus  such  outgrowths  evidently  form  embryos,  for 


50 


THE  BIOLOGY  OF  TWINS 


the  typical  number  of  embryos  in  that  species  ranges 
from  seven  to  twelve.  An  inspection  of  Fig.  15  leads 
one   to   a   different  interpretation   of   the   relation   of 


Fig.  14. — Armadillo  egg  showing  two  out  of  four  embryos  growing 
away  from  the  common  amnion  (cam).  The  other  two  embryos 
(I  and  III)  do  not  show  in  this  plate.  A  view  from  the  lower  pole  of 
the  egg  is  seen  in  Fig.  15.     (For  description  see  stage  VII.) 


secondary  to  primary  embryonic  primordia  from  that 
offered  by  Patterson.  I  am  inclined  to  believe  that 
prior  to  the  visible  formation  of  outgrowths  the  ecto- 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        51 


-dermic  vesicle  had  already  been  physiologically  differ- 
entiated into  a  number  of  radially  arranged  equivalent 
apical  points  focusing  toward  the  original  common  apex, 
and  that  those  particular  apical  points  which  hai)pened 
to  be  directed  respectively  to  the  right  and  left  sides  of 
the  uterus  had  more  room  in  which  to  grow  and  con- 
sequently developed  more  rapidly  than  those  located  in 
other  sectors,  and  thus 
became  the  primary 
embryos.  The  less 
favorably  situated 
points  that  grow  less 
rapidly  are  secondary 
and  tertiary  in  time, 
but  genetically  they  are 
as  independent  and  as 
old  as  the  primary 
embryos.  This  expla- 
nation of  the  curious 
bilateral  orientation  of 
the  quadruple  vesicle 
in  the  uterus  accords 
with  the  facts  of  devel- 
opment and  of  heredity 
better  than  others 
which  have  been  previously  offered,  and  removes,  it 
seems  to  me,  much  of  the  mystery  involved  in  the 
problems  arising  from  a  study  of  resemblances  and  of 
symmetry  relations  among  the  quadruplets. 

Stage  VIII.  The  retreat  of  the  embryos  from  the 
common  amnion  toward  the  original  animal  pole  of  the 
egg  (Fig.  16). — In  order  to  bridge  over  the  gap  between 


Fig.  15. — Outline  of  lower  pole  of 
stage  like  Fig,  14,  showing  the  four 
embryos.  The  dotted  lines  across 
are  lines  of  certain  sections  not  shown 
here.  The  four  embryonic  areas  are 
numbered  I,  II,  III,  and  IV.  (From 
Patterson.) 


52 


THE  BIOLOGY  OF  TWINS 


Tr-tl 


the  stages  shown  in  Figs.  15  and  17,  the  diagrammatic 
Fig.  16  is  shown.  Here  the  embryos,  which  are  in 
an  early  primitive-streak  stage,  have  migrated  down 
the  meridians  of  the  vesicle  and  have  left  behind 
them  thin-walled  amniotic  connecting  canals  that  still 

anchor  the  head 
ends  of  the  em- 
bryos to  the  small 
common  amnion 
{c  am)  situated  at 
the  distal  or  vege- 
tative pole  of  the 
egg.  The  posterior 
end  of  each  em- 
bryo is  now  grow- 
ing rapidly  toward 
the  rim  of  the 
Trager  ring,  a 
junction  with 
which  is  soon  to 
be  established. 

The  sectional 
view  shown  in 
Fig.  14  indicates 
that  the  heads  of 
embryos  II  and  IV  are  growing  apart,  leaving  between 
them  a  thin  sheet  of  ectoderm.  This,  together  with 
the  median  portion  of  the  original  amnion,  is  des- 
tined to  form  a  vesicle  that  remains  connected 
by  canals  with  the  amnia  of  the  individual  em- 
bryos and  is  therefore  called  the  common  amnion 
{c  am). 


11-^^ 


Fig.  16. — Armadillo  egg  showing  four 
embryos  in  early  primitive  streak  stages. 
(See  stage  VIII.)  (From  Patterson,  but 
inverted  in  order  to  be  comparable  with  the 
other  stages.) 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        53 

That  region  of  the  original  trophoblast  which  is  called 
diplotrophoblast  and  lies  at  the  distal  pole  of  the  vesicle 
and  within  the  ring  formed  by  the  fusion  of  the  endo- 
derm  and  the  trophoblast  proper  has  now  entirely  dis- 
appeared and  the  distal  portion  of  the  vesicle  is  bounded 
externally  by  endoderm.  The  point  of  fusion  between 
the  endodermic  and  trophoblastic  parts  of  the  vesicle 
wall  is  not  easy  to  detect,  but  the  endoderm  is  likely  to 
be  more  deeply  stained  in  microscopic  preparations. 

As  the  embryonic  ectoderm  grows  down  the  sides 
of  the  egg  toward  the  Trager  it  carries  with  it  the 
adjacent  endoderm,  so  that  subsequently  a  large  pro- 
portion of  the  vesicle  comes  to  be  covered  with 
endoderm.^ 

Stage  IX.  The  attachment  of  the  quadruplet  embryos 
to  the  Trager  (Fig.  17). — The  figure  is  redrawn  from  one 
published  in  1910.^  The  aim  has  been  to  represent  the 
vesicle  as  a  transparent  object,  and  this  has  been, 
partially  at  least,  realized.  In  the  previously  published 
figure  the  axis  of  the  vesicle  was  incorrectly  placed  so 
that  the  heads  of  the  embryos  were  directed  toward 
the  top  of  the  page.  In  all  previous  figures  the  original 
animal  pole  of  the  egg  is  toward  the  top  of  the  page,  and 
in  order  to  preserve  this  arrangement  with  consistency, 
all  figures  must  show  the  Trager  end  of  the  egg  at  the 

^  Reference  to  Fig.  27,  which  is  taken  from  one  of  Fernandez' 
figures  illustrating  conditions  in  the  Mulita  (D.  hybrid  us),  will  serve 
to  show  the'  extent  of  the  peripheral  endoderm.  It  also  makes  clear 
the  relations  of  the  germ-layer  components  of  the  entire  vesicle.  This 
figure  serves  equally  well  for  our  species,  D.  novcmcinctus. 

^H.  H.  Newman  and  J.  T.  Patterson,  "Development  of  the  Nine- 
banded  Armadillo  from  the  Primitive  Streak  Stage  to  B'lxih,''  Journal 
of  Morphology,  V,  21. 


54 


THE  BIOLOGY  OF  TWINS 


top  and  the  common  amnion  at  the  bottom  of  the  figure. 
This  arrangement,  doubtless,  looks  upside  down  to  one 
famihar  with  previous  accounts  of  the  embryology  of 


Fig,  17. — Armadillo  egg  with  quadruplet  embryos  attached  to 
primitive  placenta,  which  is  a  bowl-shaped  area  at  top  of  figure.  Note 
the  connecting  canals  running  downward  to  the  original  common  point 
of  origin,  which  is  now  occupied  by  the  small  common  amnion.  (See 
stage  IX.)     (Redrawn  from  Newman  and  Patterson.) 

Dasypiis,  but  the  reversal  of  axis  is  an  important  feature 
of  the  embryonic  history  and  must  be  indicated  just 
as  it  appears.  The  comparatively  smooth  outline  of 
the  Trager  region  is  due  to  the  fact  that  the  period  of 


TWINNING  IN  DASYPUS  NOVEMCINCTUS 


:):> 


Trager  nutrition  has  passed,  the  Trager  rim  has  dis- 
appeared, and  the  egg  is  beginning  to  develop  ridges 
which  act  as  physiological  drill  points  and  enable  the 
young  villi  to  penetrate  the  maternal  mucosa.  A  small 
circular  area  at  the  very  top  of  the  egg  is  comparatively 
free  of  villous  ridges  and  is  destined  to  become  a  large 
non-villous  area  such  as  is  shown  in  Fig.  19. 

Each  embryo  is  in  a  somewhat  advanced  primitive- 
streak  stage  and  has  formed  a  broad,  bandlike  connection 
with  the  margin  of  the  developing  placenta,  a  connection 
that  is  the  forerunner  of  the  umbihcus.  A  short  endo- 
dermal  allantois  opens  externally  and  extends  inward 
toward  the  placental  margin,  though  it  is  not  destined 
to  play  any  important  role  in  placentation ;  it  is  evidently 
a  vestigial  structure.  The  relation  of  this  structure 
to  the  umbilicus  is  better  shown  in  the  sectional  figure 
of  a  little  later  stage  (Fig.  26).  Each  embryo  occupies 
its  own  extra-embryonic  area  and  is  isolated  from 
corresponding  areas  of  adjacent  embryos  by  a  sort  of 
partition  resembling  the  sinus  terminalis  of  an  avian 
embryo.  The  extra-embryonic  areas  are  covered  with 
blood  islands  which  are  the  forerunners  of  the  system 
of  vitelline  blood  vessels  seen  in  the  next  figure  (Fig.  18). 

Each  embryo  has  its  own  amnion  which  is  connected 
by  its  amniotic  connecting  canal  with  the  small  com- 
mon amniotic  vesicle,  whose  origin  has  been  already 
described. 

The  two  lateral  horns  seen  in  the  figure  are  pro- 
tuberances of  the  egg  membranes  that  are  forced  into 
the  openings  of  the  paired  Fallopian  tubes.  These 
points  are  very  useful  as  a  means  of  orienting  the 
vesicle  with  reference  to  the  uterine  axes;    by  means  of 


56  THE  BIOLOGY  OF  TWINS 

these  two  points  we  are  able  to  show  that  the  arrange- 
ment of  embryos  is  not  precisely  in  accord  with  the 
uterine  bilaterality.  Embryos  II  and  IV  are  not 
exactly  lateral,  nor  are  embryos  I  and  III  strictly  ventral 
and  dorsal  respectively.  About  a  third  of  the  egg  at 
the  distal  (lower)  end  is  very  thin-walled  and  trans- 
parent, owing  to  the  fact  that  it  is  composed — except 
where  the  amnia  are  fused  with  it — of  but  two  layers  of 
cells,  endoderm  externally  and  mesothelium  internally. 
One  can  look  into  this  part  of  the  egg  as  through  a 
window. 

Stage  X.  Five-  and  seven-somite  embryos  in  an  egg 
with  early  true  placenta  (Fig.  i8). — Several  points  of 
interest  are  shown  in  this  figure,  which  is  redrawn  from 
one  previously  published  in  which  an  inadvertent  error 
was  made.^  The  placenta  now  consists  of  a  broad  band 
of  vilH  that  had  penetrated  the  mucosa  to  such  an 
extent  that  it  required  some  effort  to  pull  the  egg  free. 
The  villi  are  rather  flat  and  tend  to  overlap  like  shingles. 
Each  embryo  is  connected  with  the  placenta  by  means 
of  a  double  primitive  umbilicus,  which  as  yet  is  not 
traversed  by  blood  vessels.  The  allantois  is,  as  before, 
vestigial.  The  two  primary  embryos  (II  and  IV),  those 
nearer  the  lateral  edges  of  the  blastocyst,  are  more 
advanced  than  the  two  secondary  individuals  (I  and 
III);  this  is  due  to  the  fact  that  the  former  are  some- 
what older  than  the  latter.  The  bilateral  arrangement 
of  the  egg  is  still  preserved,  as  in  stage  IX,  by  the 
lateral  horns  or  bumps  that  represent  the  points  of 
protrusion  of  the  vesicle  wall  into  the  Fallopian  tubes. 

*  In  drawing  in  the  details  of  the  embryos  the  two  upper  embryos 
were  formerly  reversed  in  position.    This  redrawing  corrects  the  error. 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        57 

Vitelline  blood  vessels,  arranged  somewhat  as  in  the 
area  pellucida  and  area  opaqua  of  the  avian  egg,  are 


Fig.  18. — Armadillo  egg  showing  that  the  primary  embryos  (II 
and  IV)  are  in  advance  of  the  secondary  embryos  (I  and  III).  The 
primary  placenta  as  Trager  is  becoming  displaced  by  the  secondary 
true  placenta,  which  is  covered  with  villi,  or  short  finger-like  processes 
(see  stage  X).     (Redrawn  from  Newman  and  Patterson.) 

very  characteristic  features  of  this  stage;    no  blood  is 
found  in  this  vitelline  area  and  no  vitelline  circulation 


58  THE  BIOLOGY  OF  TWINS 

is  ever  established.  The  common  amnion  and  the 
amniotic  connecting  canals  still  persist  and  serve  by 
their  forked  arrangement  to  indicate  the  pairing  of 
embryos,  for  the  amniotic  canals  of  the  two  embryos 
derived  from  one  side  usually  unite  into  a  short  single 
canal  just  before  they  enter  the  common  amnion. 

Stage  XI.  A  middle-aged  egg  showing  four  discoid 
placentae  (Fig.  19). — In  order  to  show  the  embryos  and 
their  placental  connections  in  a  vesicle  of  this  stage  of 
development,  it  is  necessary  to  remove  a  part  of  the 
vesicle  wall  together  with  parts  of  the  placenta.  The 
drawing  represents  an  egg  with  a  large  window  cut  out 
from  the  median  ventral  wall.  Dotted  hues  indicate 
those  parts  of  the  two  placental  disks  which  have  been 
removed;  nothing  else  of  any  consequence  has  been 
removed.  The  entire  egg  measures  about  35  mm.  long 
and  30  mm.  wide;  each  fetus  has  a  head-rump  length 
of  about  14  mm. 

The  placenta  consists  of  four  separate  ovoid  disks 
of  treelike  villi.  The  disks  of  paired  fetuses  are  closely 
appressed  but  visibly  independent,  while  there  is  a 
distinct  space  both  dorsally  and  ventrally  between  the 
placental  disks. 

A  large  area  occupying  the  whole  upper  part  of  the 
egg  has  become  essentially  non-placental,  although 
sparse  villi  are  seen  dotted  over  its  surface,  especially 
near  the  margins  of  the  specialized  placental  areas. 

The  four  fetuses  are  seen  to  be  clearly  paired,  one 
pair  facing  toward  the  right  and  the  other  toward 
the  left;  there  is  much  more  space  between  the 
umbilical  attachments  of  unpaired  than  between  paired 
individuals. 


TWINNING  IN  DASYPUS  NOVEMCINCTUS         59 


The  upper  fetus  on  the  left  is  fetus  I,  its  i)artner  is 
II;    the  lower  right  fetus  is  III  and  its  partner  is  IV. 


Fig.  19. — An  armadillo  egg  about  six  weeks  after  fertilization, 
showing  the  two  pairs  of  fetuses,  revealed  by  the  removal  of  part  of  the 
egg  membranes.  Each  has  its  own  oval  placental  area,  its  own  amnion, 
umbilicus,  etc.  The  heavy  dotted  lines  indicate  the  boundaries  of 
the  removed  portion  of  the  placental  disks  of  the  two  nearest  embryos. 
(See  stage  XL) 

Each  fetus  has  its  own  amnion,  but  at  this  time  the 
amnia  occupy  only  a  small  part  of  the  total  volume 


6o 


THE  BIOLOGY  OF  TWINS 


of  the  blastocyst  cavity.  The  shrunken  amniotic 
connecting  canals  and  the  common  amnion  are  quite 
obvious  at  this  time  and  persist  in  stages  still  more 
advanced  than  this.  The  large  area  at  the  bottom  of 
the  vesicle,  extending  from  the  placenta  to  the  some- 
^  what  pointed  vege]- 

tative  pole,  retains  its 
transparency. 

It  may  be  noted 
that  from  this  stage  up 
to  a  stage  shortly  be- 
fore birth  no  changes 
occur  that  are  espe- 
cially significant  for  a 
study  of  twinning. 
The  whole  vesicle 
grows  enormously, 
but  the  embryo  and 
the  amnia  increase  in 
volume  more  rapidly 
than  does  the  vesicle 
wall.  The  result  is 
that  the  amnia  fill  the 
cavity  of  the  blastocyst 
as  is  shown  in  the  next 
figure. 

Stage  XII.  A  full-term  quadruplet  egg  (Fig.  20). — 
The  figure  of  the  vesicle  is  semi-diagrammatic  in  detail,^ 

^  Strahle,  in  a  paper  entitled  "tJber  den  Bau  der  Placenta  von 
Dasyurus  novemcinctus,"  gives  an  incorrect  account  of  the  arrangement 
of  the  fetuses  in  the  uterus,  in  that  he  shows  the  amniotic  partitions 
coinciding  exactly  with  the  dorsoventral  and  the  bilateral  axes  of  the 
uterus. 


Fig.  20. — Semi-diagrammatic  sec- 
tional view  of  full-term  armadillo  egg, 
seen  from  the  lower  pole  (cf.  Fig.  ig). 
The  four  fetuses  occupy  four  quad- 
rants of  the  egg,  with  partitions  be- 
tween them  composed  of  the  fused 
amnia  of  adjacent  individuals.  The 
placental  areas  are  two  thick  bilater- 
ally arranged  double  disks,  each  with 
the  umbilical  cords  of  two  fetuses 
attached.     (See  stage  XII.) 


TWINNING  IN  DASYPUS  NOVEMCLNXTUS        6i 

but  accurate  in  so  far  as  the  relation  of  placenta  and 
membranes  is  concerned.  The  view  is  one  that  would 
be  obtained  if  one  looked  into  the  vesicle  from  its  lower 
transparent  end;  the  eye  sees  the  embryos  head-on, 
so  to  speak.  The  axes  (dorsoventral  and  bilateral) 
are  the  same  as  those  of  the  mother. 

In  full-term  eggs  the  placental  complex  consists  of 
two  well-defined  areas  corresponding  to  the  right  and 
left  sides  of  the  uterus.  Those  two  heavily  villous 
areas  {p'  and  p")  are  separated,  dorsally  and  ventrally, 
respectively,  by  narrow  non- villous  areas  id  n  v  and  v  ti  v) ; 
these  serve  as  points  of  demarkation  between  the  right 
and  left  placental  regions;  they  are  not  always  precisely 
mid-dorsal  and  mid-ventral,  but  usually  slightly  as}Tn- 
metrical.  Frequently  placental  blood  vessels  run  across 
the  non-villous  area  so  as  to  connect  the  circulation  of 
the  two  sides.  A  single  fetus  may  have  parts  of  its 
placental  material  in  both  of  the  bilateral  placental 
areas.  It  appears  then  that  the  double  placenta  is 
merely  a  mechanical  adjustment  of  the  fetal  membranes 
to  the  bilateral  blood  supply  of  the  uterus. 

Although  one  fetus  appears  to  belong  to  the  right 
quadrant,  another  to  the  left,  a  third  to  the  dorsal, 
and  a  fourth  to  the  ventral,  it  is  obvious  that,  so  far 
as  placental  connections  are  concerned,  one  pair  (I  and 
II)  belongs  to  the  left  side,  the  other  pair  (HI  and  IV) 
belongs  to  the  right. 

The  umbilicus  of  each  fetus  is  close  to  an  amniotic 
partition  composed  of  the  right-hand  side  of  its  own 
amnion  and  the  left-hand  side  of  the  amnion  of  its  right- 
hand  neighbor.  Curiously  enough  there  has  been  a 
crowding  to  the  left  on  the  part  of  each  amnion  until 


62  THE  BIOLOGY  OF  TWINS 

each  one  has  filled  a  full  quadrant  of  the  vesicle  and 
has  fused  with  adjacent  amnia  wherever  contact  has 
been  estabhshed.  Why  the  pushing  over  of  the  amnia 
always  goes  toward  the  left  (anti-clockwise)  and  never 
to  the  right  (clockwise)  is  a  problem  in  developmental 
mechanics  for  which  I  have  no  solution.  Evidently, 
however,  the  vesicle  is  still  acting  as  a  unit  and  respond- 
ing to  the  same  predetermined  bias  toward  the  left  which 
was  expressed  in  the  period  of  embryonic  segregation 
when  each  primary  individual  always  pairs  with  a  sec- 
ondary individual  at  its  left  side.' 

An  egg  at  full  term  has  a  transparent  area  at  both 
proximal  and  distal  ends  and  the  broad  but  broken 
placental  zone  about  the  equatorial  region.  It  is 
readily  seen  that,  when  the  arrangement  of  fetuses  is 
so  diagrammatic,  there  can  be  no  difficulty  in  preserv- 
ing their  paired  arrangement.  One  has  merely  to  open 
the  vesicle  at  the  point  {v  n  v)  in  each  case  in  order 
to  be  able  at  any  subsequent  time  to  identify  fetuses 
I,  II,  III,  and  IV.  This  situation  is,  of  course,  highly 
favorable  for  the  study  of  correlation  and  heredity,  as 
will  presently  appear. 

Atypical  numbers  of  fetuses  in  D.  novemcinctus . — The 
description  of  embryonic  development  herewith  pre- 
sented appUes  to  a  large  proportion  of  cases.  In  about 
3  per  cent  of  cases,  however,  the  diagrammatic  relations 
typical  for  the  species  are  distorted  by  the  development 
of  more  or  less  than  the  typical  number  of  embryos. 

^  It  is  only  natural  to  refer  in  this  connection  to  the  asymmetry 
present  in  the  maturating  ovocyte  (Fig.  4),  where  the  maturation 
spindle  is  "seen  to  occur  at  one  side  of  the  formative  zone.  There  may 
be  established  here  the  basis  of  a  growth  bias  toward  the  left. 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        63 

Fernandez  has  described  and  figured  a  rare  case  in  whit  h 
a  single  fetus  occupied  an  egg  alone.  The  placental 
relations  were  decidedly  irregular  in  that  the  villous 
area  was  an  incomplete  irregular  zone  or  band  partially 
encircling  the  equator  of  the  vesicle.  It  seems  evident 
that  this  condition  is  not  due  to  a  sporadic  reversion 
to  a  uniparous  condition  but  to  the  precocious  death 
of  several  embryos.  The  zonary  position  of  the  placenta 
seems  to  require  this  interpretation,  for,  if  the  condition 
is  due  to  a  sporadic  recurrence  of  a  t>pe  of  development 
typical  for  uniparous  species  of  armadillo,  the  placenta 
should  be  discoid  and  at  the  proximal  pole  of  the  vesicle. 

In  my  own  collection  of  over  two  hundred  eggs 
there  occurred  one  case  of  twins  which  w^ere  born  in 
captivity,  and  whose  placental  relations  were  not 
determined;  two  sets  of  triplets,  in  one  of  which  unmis- 
takable traces  of  a  fourth  embryonic  rudiment  appeared ; 
and  three  sets  of  quintuplets,  in  one  of  which  an  addi- 
tional sixth  degenerating  embryonic  rudiment  was  evi- 
dent. In  all  of  these  cases  there  was  a  pronounced 
lack  of  adjustment  to  the  uterine  axes. 

One  may  therefore  conclude  that  the  quadruplet 
condition  is  practically  specific  for  D.  novcmcinctus, 
and  that  there  is  little  evidence  that  progressive  evolu- 
tion will  either  augment  or  decrease  the  typical  number. 

Variations  in  the  relations  of  embryos. — In  over  75 
per  cent  of  cases  the  paired  arrangement  of  embryos  and 
their  rather  precise  method  of  union  with  the  right  and 
the  left  placental  disks  is  like  that  described  in  con- 
nection wdth  stage  IX,  Fig.  17.  When  I  first  studied 
the  orientation  of  embryos  with  reference  to  the  uterine 
bilaterality,  I  was  much  impressed  with  the  regularity 


64  THE  BIOLOGY  OF  TWINS 

of  arrangements;  but,  after  a  careful  examination  with 
the  idea  of  finding  out  just  how  definitely  fixed  this  kind 
of  orientation  is,  I  am  surprised  to  discover  a  consider- 
able range  of  variability  in  fetal  relations. 

One  not  infrequently  finds  the  point  of  attachment 
of  fetuses  II  or  III  much  nearer  the  mid-dorsal  fine  than 
that  shown  in  Fig.  20;  similarly,  fetuses  II  or  IV  may 
be  much  nearer  the  mid-ventral  line.  In  such  cases 
the  placental  vessels  of  the  fetus  which  is  attached 
near  the  edge  of  one  or  other  placental  disk  invade  both 
right  and  left  placentae.  This  may  be  readily  shown 
by  injections. 

Cases  of  this  sort  point  to  the  conclusion  that  the 
apparent  bilaterality  of  the  blastodermic  vesicle  is 
simply  a  semblance  of  bilaterality  which  has  been 
imposed  upon  a  more  fundamental  symmetry  of  the 
vesicle  itself  by  the  uterine  blood  supply.  The  two 
heavy  placental  disks  that  develop  respectively  on  the 
right  and  left  sides  of  the  uterus  occupy  their  positions 
because  it  is  in  those  places  that  placental  growth  is 
favored  by  the  proximity  of  the  uterine  bilateral  blood 
trunks.  The  embryos  are  usually  paired  so  that  one 
pair  draws  nutriment  from  one  placental  disk  and  the 
other  pair  from  the  other  disk;  but  there  are  many 
exceptions,  as  has  already  been  indicated. 

In  previous  papers  I  have  laid  much  stress  on  the 
paired  arrangement  of  embryos  and  have  been  inclined 
to  underemphasize  those  cases  in  which  pairing  was 
absent;  a  few  cases  of  non-pairing  are  accordingly 
cited.  In  some  advanced  sets  it  is  noteworthy  that, 
when  the  ventral  bridge  between  the  two  lateral  placental 
areas  is  severed,  three  fetuses  appear  to  be  attached 


TWINNING  IN  DASYPUS  NOVEMCINCTUS        65 


to  one  large  lateral  disk  and  one  fetus  to  a  smaller  disk 
on  the  opposite  side.  Again,  in  two  sets  (Figs.  21  and 
22)  that  are  in  approximately  the  stage  shown  in  Fig.  19, 
an  interesting  irregularity  appears  in  the  arrangement 
of  the  amniotic  connecting  canals  with  reference  to  the 
common  amnion.  Instead  of  having  the  usual  dicot- 
omous  branching  on  each  side,  there  are  in  both  cases 
three  connections  on  one  side  and  a  single  unbranched 


Fig.  21 


Fig.  22 


Figs.  21  and  22. — Outline  views  of  the  common  amnion  and  the 
connecting  canals  of  fetuses.  These  were  drawn  from  two  eggs  that 
were  in  stages  between  Figs.  18  and  19,  In  Fig.  21  the  canal  of  fetus  II  is 
separated  far  from  its  partner,  fetus  I.  In  Fig.  22  fetus  IV  seems  to 
have  originated  separately  and  does  not  seem  to  be  paired  with  fetus  III. 
These  are  cases  of  non-pairing  and  may  represent  a  not  uncommon 
condition. 

connection  on  the  other.  Doubtless  if  a  larger  collection 
of  equivalent  stages  were  available  other  similar  con- 
ditions would  be  revealed.  This  departure  from  the 
symmetrical  paired  arrangement  is  significant  when 
compared  with  what  Fernandez  describes  for  the 
Mulita,  where  the  irregular  condition  is  the  rule  and 
there  is  little  evidence  of  pairing  (compare  Figs.  27-31). 


66  THE  BIOLOGY  OF  TWINS 

One  might  have  been  led  to  suspect  the  occurrence 
of  exceptions  to  the  rule  that  the  embryos  of  D.  novem- 
cinctus  consist  of  two  pairs,  one  derived  from  the  right 
and  one  from  the  left  primary  ectodermic  outgrowth, 
for,  long  before  the  information  just  given  was  avail- 
able, it  was  known  that  the  inter-resemblances  of  quad- 
ruplet sets  were  occasionally  out  of  accord  with  the 
paired  arrangement.  Sometimes  three  out  of  four 
fetuses  were  alike  in  the  possession  of  some  one  genetic 
feature  and  the  fourth  was  different;  similarly,  it  was 
occasionally  noted  that  but  one  fetus  possessed  a 
peculiarity  and  the  other  three  were  without  it.  At 
the  risk  of  anticipating  the  conclusions  expressed  in  a 
subsequent  chapter  I  may  say  that,  were  we  in  posses- 
sion of  the  facts  as  to  the  origin  of  the  four  quadruplets 
from  the  common  ectodermic  vesicle,  all  of  the  data 
on  resemblances,  which  are  to  receive  treatment  sub- 
sequently, would  be  completely  rationalized. 

DEVELOPMENT  OF  THE  PLACENTA 

The  history  of  the  placenta  is  of  considerable  impor- 
tance for  an  understanding  of  twinning.  It  gives  us 
the  criteria  for  distinguishing  the  four  embryos  and 
their  paired  or  non-paired  relationships  even  in  the 
last  stage  of  uterine  development.  A  brief  resume 
of  the  facts  relating  to  the  placenta  will  accordingly 
insure  a  clearer  understanding  of  what  is  to  come. 
The  primary  placenta  is  the  Trager,  a  ring  of  specialized 
trophoblast  that  forms  the  first  connection  with  the 
uterine  mucosa.  This  Trager  subsequently,  as  in 
Fig.  17,  becomes  uniformly  studded  with  adhesion 
pads   destined   to   become   the  burrowing   tips   of   the 


TWINNING  IN  DASYPUS  NOVEMCTXCTUS         67 

definitive  villi.  Later,  when  the  four  embryos  form 
an  attachment  at  their  posterior  part  with  the  IVa^^er 
ring,  a  co-operation  between  embryonic  endodcrm 
and  the  Trager  epithelium  takes  place,  resulting  in  the 
formation  of  hollow  vascular  vilh  that  invade  deeply 
the  maternal  mucosa.  Four  separate  patches  of  villi 
appear  corresponding  to  the  point  where  each  embryo 
has  developed  its  own  nutritive  attachment  with  the 
mother.  With  the  rapid  peripheral  extension  of  those 
villous  patches  there  is  an  apparent  fusion  of  the  four 
placentae  into  a  scalloped  placental  ring  which  appears 
to  be  continuous,  but  is  not,  for  there  is  no  admixture 
of  blood  between  adjacent  fetuses.  This  has  been 
fully  demonstrated  by  injections.  This  ''compound 
zonary  placenta,"  as  it  has  been  called,  remains  as  an 
apparently  complete  ring  about  the  equator  of  the 
vesicle  until  rather  late  stages  of  development.  Then 
there  follows  a  separation  of  the  ring  into  two  almost 
separate  discoid  placentae,  each  of  which  receives  the 
umbilical  cords  of  a  pair  of  embryos.  If  one  examine 
a  gravid  uterus  during  the  last  month  of  pregnancy,  he 
will  readily  note  that  in  the  median  dorsal  and  median 
ventral  regions  there  is  an  area  almost  devoid  of  pla- 
cental tissue.  If  the  uterus  be  cut  open  along  this  clear 
Hne  on  the  ventral  side,  the  cut  will  fall  between  the 
placental  disks  of  the  two  halves  of  the  uterus,  and  the 
orientation  of  pairs  will  usually  be  preserved. 


CHAPTER   III 

MODES  OF  TWINNING  IN  OTHER  SPECIES 

OF  ARMADILLO 

Before  attempting  a  discussion  of  the  probable 
origin  and  causes  of  polyembryonic  twinning  in  the 
nine-banded  armadillo  it  will  be  well  to  learn  something 
about  the  conditions  known  for  other  species  of  arma- 
dillo. Nothing  is  more  likely  to  furnish  a  clue  to  the 
solution  of  the  problems  presented  by  one  particular 
species  than  a  comparative  study  of  the  embryology  of 
allied  forms. 

Previous  references  have  been  made  to  Fernandez' 
account  of  the  polyembryonic  development  of  the 
Mulita  (Dasypus  hyhridus).  His  account  of  stages 
corresponding  to  those  illustrated  by  our  Figs.  12-19 
are  so  nearly  identical  with  those  described  in  the  last 
chapter  for  D.  novemcinctus  that  it  will  be  unnecessary 
to  do  more  than  indicate  the  somewhat  minor  differences 
in  detail.  A  comparison  of  Fernandez'  figures  (Figs.  23 
and  24)  with  our  Figs.  12  and  14  will  indicate  the  close 
similarity. 

The  two  species  of  Dasypus  are  evidently  very 
closely  related,  and  it  would  appear  probable  that  D.  hy- 
hridus is  a  comparatively  recent  derivative  of  D.  novem- 
cinctus. The  only  important  particular  in  which  a 
difference  exists  is  in  the  number  of  young  in  a  poly- 
embryonic litter.  While  in  D.  novemcinctus  the  number 
is  almost  uniformly  four,  in  D.  hyhridus  the  number  is 

68 


TWINNING  IN  OTHER  SPECIES  OF  ARMADILLO     69 

larger  though  much  less  definitely  fixed.  Hicre  is  a 
strong  tendency,  however,  for  the  species  to  settle  down 
upon  the  number  eight,  though  litters  of  nine  are  fre- 
quent and  from  seven  to  twelve  are  reported. 


Fig.  23 


Figs.  23  and  24. — Diagrammatic  views  of  two  stages  in  the  develop- 
mant  of  Dasypus  hybridus  (the  Mulita  armadillo).  For  comparison 
with  equivalent  stages  of  D.  novemcinctus  (Figs.  12  and  14)  they  should 
be  viewed  inverted.  Note  Trager  cavity  (trcav),  trophoderm  jilate 
(tr  pi),  ectoderm  (ec),  endoderm  (en),  mesoderm  (ms),  diplotrophoblast 
(dtr),  extra-embryonic  cavity  {ex  c),  uterine  mucosa  {mucut),  common 
amnion  {c  am),  primitive  streak  of  embryos  {pr  st),  amniotic  connecting 
canal  {en  am).     (From  Fernandez.) 

The  fact  that  there  is  so  much  variability  in  the 
number  of  polyembryonic  offspring  in  this  species 
apparently  indicates  that  the  condition  is  of  com- 
paratively recent  origin.  This  view  is  supported  by 
the  fact  that  so  large  a  percentage  of  embryos  in 
advanced  stages  are  dead  or  show  signs  of  marked 
abnormality,  owing  to  overcrowding  and  ill-success  in 


70 


THE  BIOLOGY  OF  TWINS 


the  struggle  for  placental  surface.  What  evidently 
happens  is  that  four  or  more  secondary  growing  points 
start  to  develop  simultaneously,  instead  of  the  two 
that  are  characteristic  of  D.  novemcinctus,  and  that 
normally  each  of  these  growing  points  divides  into  the 
primordia  of  two  embryos;  but  sometimes  more  than 
two  embryos  are  the  result  of  this  fission  and  sometimes 


Pi 

^ 

>^t^ 

^ 

S 

k^ 

r  w 

fe"?^!^^   -,, 

'     '^^ 

^ 

' «' 

m4 

^^HK^gjI^'  ".'      4 

Fig.  25. — Photographic  view  of  a  set  of  embryos  of  D.  hyhridiis 
(after  Fernandez).  Note  the  common  amnion  in  the  middle  and  the 
amniotic  connecting  canals  running  to  the  nine  embryos. 

no  fission  occurs.  Such  irregularities  as  these  are  similar 
to  the  formation  in  D.  novemcinctus  of  three  embryos 
instead  of  the  typical  two  from  one-half  of  the  ectoder- 
mic  vesicle,  resulting  in  five  embryos.  That  the  above 
interpretation  of  the  origin  of  the  number  of  fetuses  in 
D.  hyhridus  is  probably  correct  may  be  inferred  from  an 
examination  of  Fernandez'  photographs;  Fig.  25  is 
taken   from   one   of    these.     Note    that    the   amniotic 


TWINNING  IN  OTHER  SPECIES  OF  ARMADILLO     71 

canals  are  forked  just  as  are  those  of  a  pair  of  embryos 
of  D.  novemcinctus  that  come  from  a  primary  outgrowth, 
a  fact  that  lends  probability  to  the  view  that  the  develop- 
ment of  polyembryony  in  the  two  si)ecies  of  Dasypus 
is  practically  identical  in  character.  It  should  also 
be  said  that,  even  in  sets  with  eight  or  more  fetuses, 
there  is  no  exception  to  the  rule  that  all  from  a  single 
egg  are  of  the  same  sex.  Unfortunately  nothing  is 
known  about  the  heredity  of  armor  characters,  nor 
about  the  symmetry  relations  existing  between  the 
different  members  of  a  polyembryonic  set.  Such  a 
study  of  the  species,  if  correlated  with  what  has  been 
published  in  these  connections  about  D.  novemcinctus, 
would  be  important. 

We  are  indebted  to  Fernandez  for  a  clear  under- 
standing of  the  interrelations  of  germ-layers  and  of  the 
peculiarities  of  amnia  and  allantois  in  D.  hyhridus. 
The  diagram  (Fig.  26)  is  adapted  from  Fernandez' 
first  paper.  It  would  serve,  however,  almost  equally 
well  for  D.  novemcinctus.  Only  two  embryos  that  lie 
to  right  and  left  of  the  egg  are  shown.  Note  the  external 
endoderm  {en),  the  rudimentary  allantois  (j/),  and  the 
short  yolk  stalk  opening  into  the  yolk  sac,  which  is  in 
this  case  inverted  so  as  to  form  the  external  layer  of 
the  vesicle.  The  belly-stalk  {bs)  or  primitive  umbilicus 
is  estabhshing  a  relation  with  the  Trager,  but  as  yet 
no  umbilical  vessels  have  invaded  it.  A  posterior 
prolongation  of  the  amnion  {p  am)  goes  back  toward 
the  Trager,  but  appears  to  have  no  part  in  the  formation 
of  an  umbiHcus.  The  amniotic  connecting  canals  and 
common  amnion  are  showm  wath  both  ectodermal  and 
mesodermal  layers  present. 


72 


THE  BIOLOGY  OF  TWINS 


Five  years  after  his  first  paper  on  polyembryony  in 
D.   kyhridiis  Fernandez'^  reported  further  facts  about 


Fig.  26 


this  species  and  dwelt  especially  upon  the  origin  of  the 
individual  embryos  from  the  undivided  egg.     He  appears 

^  M.  Fernandez,  Proc.  IX^  Congres.  de  Zobl.  Monaco,  19 14. 


TWINNING  IN  OTHER  SPECIES  OF  ARMADILLO     73 

to  have  adopted  the  "budding"  hypothesis  as  an  expla- 
nation of  the  mode  of  isolation  of  the  several  embryos 
from  the  ectodermic  vesicle,  since  he  uses  the  word 
''Sprossung"  to  describe  the  process.  Possibly,  however, 
this  term  may  merely  imply  evagination  or  outgrowth, 
and  thus  be  free  from  the  unfortunate  implication  carried 
by  the  word  '' budding."  In  brief,  this  is  Fernandez' 
idea  of  the  mode  of  polyembryonic  development  in 
D.  hybridus:  the  entire  ectoderm  of  the  individual 
embryos  originates  from  the  undivided  primary  ectoder- 
mic vesicle  through  a  series  of  irregular  and  complicated 
outgrowths,  while  the  endoderm  and  mesoderm  of  each 
embryo  is  produced  in  loco  from  the  common  mesoderm 
at  points  where  the  outsprouting  ectoderm  comes  in 
contact  with  them.  This  haphazard  method  of  origin  of 
the  embryos  makes  it  clear  then  why  the  number  of 
embryos  in  the  Miilita  is  not  fixed  but  so  highly 
variable. 

This  explanation  of  polyembryony  is  purely  descrip- 
tive and  implies  no  theory  as  to  the  factors  responsible 
for  the  condition.  Some  of  Fernandez'  figures  of  the 
common  amnion  and  the  interrelations  of  the  amniotic 
connecting  canals  seem  to  indicate  that  not  all  of  the 
outgrowths  of  the  ectodermic  vesicle  are  primary,  but 
that  some  of  them  are  secondary  or  even  tertiary.  In 
one  case  (Fig.  27)  there  appear  to  be  four  independent 
primary  outgrowths,  each  connected  separately  with 
the  common  amnion,  and  one  compound  outgrowth, 
consisting  of  four  branches,  one  of  which  is  much 
smaller  than  the  rest.  In  another  case  (Fig.  30)  the 
common  amnion  seems  to  have  divided  into  two  vesicles 
united  by  a  narrow  neck.     From  one  half-vesicle  seven 


74 


THE  BIOLOGY  OF  TWINS 


Fig.  27. — Drawing  of  a  wax  model  of 
the  ectodermic  vesicle  of  an  egg  of  D. 
hybridus,  showing  the  relationship  of  the 
nine  embryonic  outgrowths.  Note  the 
great  irregularity  and  lack  of  definite 
pairing,     (From  Fernandez.) 


embryos  have  arisen;    from  the  other  only  one  rudi- 
mentary or  degenerate  embryo.     Other  arrangements  of 

embryos  are  shown 
in  Figs.  28  and  29. 

This  highly  vari- 
able condition  in  the 
Mulita  is  in  sharp 
contrast  with  the 
rather  definite  one 
that  prevails  in  D. 
novemcinctus,  where 
about  97  per  cent  of 
sets  have  four  fetuses. 
One  cannot  help 
suspecting,  however, 
that,  as  was  previ- 
ously suggested,  the 
quadruplet  condition 
in  the  last-named 
species  is  not  always 
arrived  at  in  the 
same  way.  Just  as  in 
the  Mulita  one  sprout 
may  remain  single 
and  another  sub- 
divide once  or  several 
times,  so  in  our  spe- 
cies it  may  well  be 
that  quadruplets 
arise,  not  always  in 
pairs,  but  sometimes  three  on  one  side  and  one  on  the 
other.     This  would  furnish  an  explanation  for  the  con- 


FiG.  28. — Showing  the  same  region  of 
D.  hybridus  that  is  shown  for  D.  novem- 
cinctus in  Figs.  21  and  22.  Note  the  very 
irregular  interrelations  of  the  connecting 
canals  of  the  embryos  yl  to  /.  C  connects 
with  a  rudimentary  embry^o.  A  and  B 
might  be  called  a  pair;  similarly  D  and  E. 
(From  Fernandez.) 


TWINNING  IN  OTHER  SPECIES  OF  ARMADILLO     75 


Fig.  29 


dition  that  occasionally  appears  in  which  three  fetuses 
are    alike  and  one 
quite  different. 

A  striking  feature 
of  Fernandez'  collec- 
tion   of    the    eggs  of 


D.  hyhridus  has  to  do 
with  the  frequent, 
almost  universal, 
occurrence  of  one  or 
more  degenerate 
embryos  in  an  egg. 
These  embryos  may 
be  the  victims  of 
severe  competition 
for  placental  surface, 
or  they  may  be  the 
result  of  outgrowths 
produced  from  un- 
favorable regions 
of  the  ectodermic 
vesicle.  The  portion 
of  an  Qgg  shown  in 
Fig.  31  indicates 
that  ectodermal  out- 
growths may  fail  to 
reach  the  walls  of  the 
egg  and,  through  lack 
of  endodcrmic  and 
mesodermic  elements, 
may  thus  fail  to  be- 
come   complete    em- 


FlG.   30 

Figs.  29  and  30.  Two  more  views  of 
conditions  like  those  shown  in  Fig.  28. 
There  are  in  the  upper  figure  13  embryos, 
each  with  a  connecting  canal;  .1,  B,  and 
C  are  evidently  rudimentary  or  aborted 
embryos.  In  the  lower  figure  all  of  the 
successful  embryos  (1-7)  seem  to  have 
come  from  one-half  of  the  ectodermic 
vesicle  and  only  a  rudimentary  embryo 
(8)  from  the  other  half.  (After 
Fernandez.) 


76 


THE  BIOLOGY  OF  TWINS 


bryos.     They  would  also  never  establish  a  nutritive  con- 
nection with  the  placenta. 

A  drawing  of  a  wax  model  (Fig.  27)  made  from  an 
ectodermic  vesicle  from  which  numerous  embryonic 
outgrowths  are  being  given  off  shows  two  outgrowths 
that  give  promise  of  rudimentation.  Embryonic  out- 
growth 9  has  evidently  been  produced  too  high  up  or 
too  close  to  the  original  apical  end  of  the  vesicle  and 


Fig.  31. — A  group  of  primitive  streak  embryos  of  D.  hyhridus  show- 
ing how  some  of  the  ectodermic  outgrowths  {F  and  H)  fail  to  secure 
attachment  to  the  endodermic  parts  of  the  egg.  This  may  account 
for  the  rudimentary  embryos  seen  in  Figs.  29  and  30.     (After  Fernandez.) 

has  not  been  able  to  grow  out  as  rapidly  as  the  rest; 
embryo  2  is  small  and  apparently  subsidiary  to  i  and 
would  probably  have  been  rudimentary.  Rudimentary 
embryos  that  are  no  more  than  formless  masses  of 
tissue  attached  by  amniotic  connecting  canals  with 
the  common  amnion  are  shown  in  Fig.  29.  Here  also 
are  shown  marked  irregularities  in  the  interrelations 
of  embryos,  as  indicated  by  the  forking  of  amniotic 
canals. 


TWINNING  IN  OTHER  SPECIES  OF  ARMADILLO     77 

A  good  many  cases  of  rudimentary  embryos  have 
been  found  in  D.  novcmcinctiis,  but  the  mortality  of 
embryos  in  that  species  is  very  low. 

TWINS    IN    EUPHRACTUS  VILLOSUS^ 

Late  in  the  year  19 15  there  appeared  a  third  paper 
by  Fernandez^  which  gives  important  information  about 
the  development  of  the  hairy  armadillo  or  Peludo 
{Euphractus  villosus)  ? 

Euphractus  possesses  a  mode  of  development  strik- 
ingly different  from  that  of  Das y pus  and  a  new  and 
totally  different  kind  of  twinning.  When  one  examines 
the  advanced  embryonic  vesicle  of  this  species,  he  finds 
a  situation  like  that  shown  in  Fig.  32.  Usually  two 
fetuses  are  present  within  what  appears  to  be  a  single 
chorion,  and  they  are  separated  from  one  another  by  a 
membrane  that  appears  to  be  made  up  of  the  fused 
amnia  of  the  two  fetuses.  Each  fetus  has  its  own 
separate  umbilicus,  and  the  two  are  not  fastened  to  the 
wall  of  the  vesicle  with  any  regard  to  the  uterine  axes  of 
symimetry.  Fernandez  examined  ten  advanced  vesicles 
and  found  that  in  seven  cases  the  twin  fetuses  were 
of  opposite  sexes;  in  two  cases  both  were  female  and  in 
one  case  both  were  male.  The  occurrence  of  fetuses  of 
opposite  sexes  seemed  strange  in  the  case  of  twins  that 
had  every  appearance  of  being  monochorial;  hence  the 
situation  deserved  careful  investigation, 

^  This  species  has  been  incorrectly  attributed  by  Fernandez  to  the 

genus  Dasypus. 

^M.  Fernandez,  Anat.  Anzcigcr,  Bd.  48,  No.  13/14,  i9i5- 

3  The  genera  Euphractus  and  Dasypus  are  by  some  authors  placed 

under  one  genus,  and  not  without  some  show  of  reason,  for  the  species 

of  the  two  genera  are  certainly  very  similar. 


78 


THE  BIOLOGY  OF  TWINS 


A  considerable  collection  of  34  embryonic  vesicles 
was  assembled,  showing  stages  from  the  primitive  streak 
stage  up  to  a  stage  of  advancement  similar  to  that 
shown  in  Fig.  32.      The  net  result  of  the  analysis  of 


Fig.  32. — Photograph  (after  Fernandez)  of  a  double  embryonic 
vesicle  of  the  armadillo  Euphractiis  {Dasypus)  villosus,  showing  the  two 
fetuses  inclosed  in  what  appears  to  be  a  common  chorion  and  separated 
by  an  amniotic  partition  composed  of  the  fused  amnia  of  the  twins. 
These  come  from  two  eggs  and  may  be  or  may  not  be  of  opposite  sex. 

this  material  was  that  the  twins  proved  not  to  be 
derived  from  one  egg  (i.e.,  were  not  monozygotic)  at 
all.  The  false  appearance  of  polyembryony  was  due 
to  the  very  intimate  fusion  of  two  originally  quite 
separate  eggs,  which  were  merely  crowded  so  closely 


TWINNING  IN  OTHER  SPECIES  OF  ARMADILLO     79 

in  the  small,  simple  uterus  that  their  external  mcm])ranes 
fused  together  so  as  to  simulate  a  monochorial  condition. 
It  is  certainly  a  strange  circumstance  that  within  a 
small  group  of  closely  related  and  sharply  differentiated 
forms  like  the  armadillos  there  should  occur  two  methods 
of  twinning  so  diametrically  opposite  in  character: 
the  splitting  up,  as  in  Dasypns,  of  a  single  egg  into 
several  embryos,  and  the  intimate  fusion,  as  in  EupJirac- 
tus,  of  originally  separate  eggs  into  one  vesicle.  Stranger 
still  is  the  fact  that  the  end-results  of  the  two  processes 
are  so  strikingly  similar  as  to  lead  one  to  believe  that 
they  are  the  result  of  the  same  fundamental  processes. 
Such  a  finding  as  this  should  indicate  the  necessity  of 
caution  in  interpreting  the  various  types  of  monochorial 
conditions  in  human  twdns,  for  it  seems  highly  probable 
that  the  common  chorion  in  many  observed  cases  may, 
as  in  Euphractus,  be  due  merely  to  a  secondary  fusion  of 
originally  separate  membranes. 

Another  interesting  discovery  is  that  in  Euphractus 
mllosus  occasional  cases  of  uniparous  births  occur. 
Fernandez  found  five  such  cases  in  thirty-four  pregnant 
uteri.  Three  interpretations  of  this  condition  are 
obviously  possible.  There  may  be:  {a)  a  certain 
amount  of  prenatal  mortality  of  one  twin  of  a  pair; 
ih)  a  failure  to  ovulate  on  the  part  of  one  of  the  ovaries; 
or  {c)  one  of  the  eggs  may  fail  to  be  fertihzed.  Other 
reasons  might  be  suggested,  but  since  Fernandez  does 
not  furnish  the  crucial  data  necessary  for  deciding  the 
matter — a  statement  as  to  whether  in  these  cases  of 
single  embryos  one  or  two  corpora  lutea  occur — it 
would  be  useless  to  speculate  further.  It  seems  strange 
that  one  who  knows  so  well  the  unique  value  of  the 


8o 


THE  BIOLOGY  OF  TWINS 


corpus  luteum  as  a  means  of  distinguishing  between 
multiple  births  and  polyembryony  should  omit  to  furnish 
this  information. 


allaiL 


p.  ,•/ 


dtA 


Fig.  33 


Fig.  34 


Figs.  33  and  34. — ^Two  stages  in  the  development  of  the  egg  of  the 
armadillo  Euphractus  {Dasypus)  villesus  (from  Fernandez),  showing 
how  each  of  the  individuals  shown  in  Fig.  32  appears  before  they  fuse 
membranes.  In  Fig.  7^:^  the  single  embryo  has  formed  at  the  lower  pole, 
as  in  Dasypus  (cf.  Figs.  13  and  14),  but  only  one  primitive  streak 
arises.  In  Fig.  34  the  single  embryo  has  grown  backward,  as  in  Dasypus, 
to  the  upper  pole,  where  placentation  has  occurred. 

The  Peludo  shows  us  typical  armadillo  development 
unobscured  by  polyembryonic  processes,  and  on  that 
account  should  throw  light  on  the  mechanics  of  poly- 
embryony. A  comparison  of  the  earliest  known  stages 
of  the  development  of  the  Peludo  and  those  in  Dasypus 
may  be  made  by  examining  the  diagram  of  Fernandez 


TWINNING  IN  OTHER  SPECIES  OF  ARMADILLO     8i 

(Fig.  33)  in  connection  with  Figs.  12  to  14  for  Dasypus. 
In  both,  through  the  process  of  so-called  germ-layer 
inversion,  the  ectodermic  vesicle  is  at  the  distal  i)ole  of 
the  fixed  vesicle  and  the  endodermic  vesicle  covers  the 
external  part  of  the  vesicle  from  the  Trager  downwarrl. 
Evidently  then  this  process  of  germ-layer  inversion  is 
not,  as  was  at  first  supposed,  the  key  to  polyembryonv. 
It  merely  furnishes  the  conditions  under  which  polv- 
embryonic  development  may  easily  occur.  Note  that 
in  Euphr actus  the  Trager  is  in  the  form  of  a  vesicle  with 
a  completely  inclosed  cavity  called  the  Trager  cavity. 
In  D.  novemcinctiis  there  appears  to  be  no  true  Trager 
cavity,  for  the  proximal  wall,  corresponding  to  that 
at  the  top  of  the  Fernandez  figures,  does  not  develop. 
The  trophoderm  plate  is  the  same  in  both  species,  and 
the  thickened  ring  of  trophoderm  (the  Trager)  that  in 
Dasypus  invades  the  maternal  mucosa  in  stages  rei^ re- 
sented in  Figs.  12  to  16  corresponds  to  the  thickened 
side  walls  of  the  trophodermic  vesicle  in  Euphractus. 
The  difference  in  the  two  species  is  associated  with  the 
fact  that  in  Euphractus  the  trophodermic  vesicle  does 
not  sink  deeply  into  the  maternal  mucosa  but  lies  more 
on  the  surface,  while  in  Dasypus  the  penetration  of 
the  Trager  into  the  maternal  mucosa  is  much  deeper, 
involving  a  practically  complete  dropping  of  the  proxi- 
mal wall  of  the  Trager  cavity.  The  sides  of  this  vesicle 
persist  as  an  ingrowing  ring  or  collar  of  glandular  cells 
that  penetrate  rather  deeply  into  the  mucosa  epithelium ; 
but  the  trophodermic  plate  inclosed  by  this  ring  remains 
free  from  the  mucosa  and  may  sometimes  have  between 
it  and  the  underlying  mucosa  a  considerable  space  which 
may  be  homologized  with  the  Trager  cavity  of  Fernandez. 


82  THE  BIOLOGY  OF  TWINS 

Although  the  embryo,  now  represented  mainly  by  the 
ectodermic  vesicle,  at  first  lies  at  the  distal  pole  of  the 
egg,  it  subsequently  shifts  its  position  until  it  appears 
to  be  attached  to  the  proximal  pole  of  the  egg,  as 
shown  in  Fernandez'  figure.  The  explanation  of  this 
apparent  total  reversal  of  position  is  not  far  to  seek,  for 
there  is  a  close  analogy  in  this  respect  between  Dasypus 
and  Euphractus.  Referring  again  to  Fernandez'  figure 
(Fig.  34) ,  we  see  that  the  Haftstiel  or  primitive  umbilicus 
shows  the  same  relation  to  the  endodermal  allantois 
that  exists  in  the  genus  Dasypus.  The  embryo  appears 
to  have  grown  backward  along  the  vesicle  wall  through 
an  arc  of  over  180  degrees.  The  result  is  that  the 
anterior  end  of  the  embryo  which  formerly  pointed  to 
the  left  now  points  to  the  right;  the  dorsal  aspect,  that 
formerly  faced  toward  the  proximal  pole,  now  faces 
toward  the  distal  pole.  Note  the  slender  Nabelstrang  in 
Fig.  34,  running  from  the  junction  of  amnion  and  umbil- 
icus to  the  distal  pole  of  the  vesicle,  which,  I  believe,  is 
the  equivalent  of  the  amniotic  connecting  canals  in 
Dasypus.  Analysis  of  the  situation  reveals  the  impor- 
tant fact  that  each  quadruplet  embryo  in  Dasypus  goes 
through  the  same  reversal  of  axis,  the  same  backward 
growth  toward  the  Trager,  and  the  same  establishment 
of  a  placental  connection  with  the  latter  as  does  the 
single  embryo  of  Euphractus.  Starting  with  a  single 
ectodermic  vesicle  in  both  species,  in  Euphractus  a  single 
apical  end  and  embryonic  axis  is  established,  and  in 
Dasypus  four  or  more  apical  ends  appear.  The  real 
problem  of  polyembryony  is  to  account  for  the  appear- 
ance of  four  growing  points  in  a  vesicle  that  primitively 
had  but  one. 


TWINNING  IN  OTHER  SPECIES  OF  ARMADILLO     83 

There  is  evidence  that  En phr actus  villosiis  ma>'  be 
evolving  toward  a  uniparous  condition,  for  single  fetuses 
occur  in  about  15  per  cent  of  the  cases  observed  by 
Fernandez.  This  author  also  observes  that  the  uterus 
of  a  young  female  is  distinctly  bicornate  in  structure,  a 
fact  that  may  serve  as  evidence  that  multiple  gestation 
was  the  primitive  condition  and  that  the  occurrence 
of  a  single  birth  is  a  modern  tendency  resulting  from  the 
gradual  transformation  of  the  uterus  from  the  primitive 
bicornate  form  into  the  simple  form. 

Two  closely  allied  species  of  Dasypus,  D.  sexcinctus 
(the  six-banded  armadillo)  and  D.  gymnarus  (the  one- 
banded  armadillo),  have  been  described  by  von  Kolliker 
and  by  Chapman  as  normally  uniparous,  but  Carson  in 
a  letter  states  that  he  had  a  female  D.  sexcinctus  in  the 
Philadelphia  Zoological  Garden  that  in  three  successive 
pregnancies  produced  twins  twice  and  a  single  fetus 
once.  Evidently  then  twinning  is  quite  common 
among  the  armadillos  and  is  probably  the  normal 
condition  in  many.  In  the  Httle  three-banded  arma- 
dillo, Tolypeutes  conurus,  the  uniparous  condition  is 
evidently  typical,  as  Fernandez  says  of  it:  ''AUe 
trachtigen  uteri  des  Mataco  (Tolypeutes  conurus)  fiihren 
nur  einen  Embryo."  He  does  not  state,  however, 
upon  how  many  cases  this  statement  is  based. 

One  may  infer  then  that  the  earliest  ancestors  of 
the  modern  armadillos  had  bicornate  uteri  and  had 
multiple  offspring;  that  the  next  step  was  a  shortening 
of  the  horns  and  a  tendency  to  produce  twins;  that 
with  the  development  of  a  simplex  uterus  there  came  a 
tendency  to  produce  single  offspring,  which,  in  some 
species  has  become  a  specific  character.     In  the  genus 


84  THE  BIOLOGY  OF  TWINS 

Dasypus,  which  is  evidently  the  most  highly  specialized 
genus  of  the  Dasypodidae,  the  process  of  polyembryony 
is  the  last  step  and  has  been  derived  from  a  uniparous 
situation  through  the  introduction  of  some  factor  that 
causes  the  single  blastocyst  to  undergo  precocious 
agamic  reproduction.  Polyembryony  is,  therefore,  not 
a  primitive  but  a  highly  specialized  condition,  though 
some  authors  refer  it  back  to  conditions  in  the  lower 
chordates  or  even  to  conditions  in  the  invertebrates. 


CHAPTER  IV 

THEORIES  OF  POLYEMBRYONIC  DEVELOPMENT 

IN  DASYPUS 

A  clue  to  a  physiological  explanation  of  polyembry- 
ony  appears  to  be  presented  by  a  comparison  of  the 
developmental  rates  of  uniparous  and  of  polyembryonic 
armadillos.  In  both  species  of  Dasypus  the  gestation 
period  is  abnormally  long  for  a  mammal  of  such  com- 
paratively small  size,  covering  a  period  of  from  four  to 
five  months;  in  Euphradus  villostis,  in  which  one  egg 
gives  rise  to  but  one  embryo,  the  period  of  gestation  is 
only  two  months.  So  short  is  the  gestation  period  that 
two  broods  are  produced  in  a  season  instead  of  one,  as 
in  Dasypus.  Fernandez  attempts  to  explain  the  slower 
development  of  Dasypus  by  saying  that  the  nutriment 
received  by  the  embryos  from  the  maternal  blood  is 
not  sufhcient  to  allow  development  to  proceed  at  the 
accustomed  rate,  and  that  the  crowding  of  many 
fetuses  in  a  simple  uterus  tends  further  to  retard  develop- 
ment. He  forgets,  however,  that  polyembryonic  devel- 
opment begins  and  is  fully  completed  long  before  the 
young  egg  establishes  a  permanent  nutritive  rehition 
with  the  maternal  tissues  and  that  there  is  no  crowding 
in  the  uterus  until  the  completely  formed  embryos  have 
attained  a  considerable  degree  of  advancement.  We 
cannot,  therefore,  explain  the  establishment  of  four 
or  more  growing  points  at  the  apex  of  the  ectodcrmic 
vesicle  as  due  to  insufficient  maternal  nutriment  or  to 
crowding. 

8S 


86  THE  BIOLOGY  OF  TWINS 

A  much  more  satisfactory  explanation  is  associated 
with  the  fact  that  there  is  an  early  "period  of  quies- 
cence" of  the  egg.  This  fact,  though  no  significance  was 
attributed  to  it,  was  brought  out  by  Patterson,  who 
found  that  all  of  the  eggs  collected  over  a  period  extend- 
ing from  the  middle  of  October  to  the  fourth  of  Novem- 
ber were  in  almost  the  same  stage  of  development.  It 
was  found,  moreover,  that  eggs  in  the  Fallopian  tubes 
were  almost  as  advanced  as  those  found  practically  in 
the  area  of  placentation.  These  observations  prove 
that  the  factors  responsible  for  retardation  of  develop- 
ment in  the  polyembryonic  species  are  in  operation  at 
a  very  early  period,  probably  as  early  as  the  first  cleavage 
stages.  The  problem  is  to  locate  the  factors  respon- 
sible for  the  slowing  down  of  the  developmental  rhythm. 
Whatever  these  factors  may  be,  and  we  have  no  definite 
knowledge  of  them,  the  result  of  retardation  is  poly- 
embryony. 

For  some  years  I  have  been  convinced  that  this 
case  of  agamic  reproduction  is  physiologically  equiva- 
lent, in  some  important  respects,  to  the  well-known 
case  of  budding  in  plants.  This  view  was  expressed 
in  19 13  in  a  general  paper^  on  the  natural  history  of  the 
nine-banded  armadillo.  In  that  paper  I  ventured, 
perhaps  unwisely,  to  attribute  the  retarded  develop- 
ment to  the  presence  of  a  specific  parasite  in  the  arma- 
dillo egg.  This  structure  had  at  that  time  been  diag- 
nosed for  me  by  an  expert  protozoologist  as  a  sporozoan 
parasite.  At  the  present  time  I  am  unable  to  decide 
whether  the  object  in  question  is  a  protozoan  or  not; 
at  least  it  is  of  universal  occurrence  in  the  armadillo  egg, 

^  H.  H.  Newman,  American  Naturalist,  XLVII,  1913. 


THEORIES  OF  TOLYEMBRYONIC  DEXELOPMEXF     87 

and  is  not  found  in  the  eggs  of  other  mammals.  Whether 
the  structure  is  the  cause  of  retarded  development,  or 
merely  one  of  its  results,  cannot  be  decided  at  pres- 
ent. In  the  paper  just  referred  to  I  made  the  fol- 
lowing statement,  which  deserved,  I  believe,  furlher 
elaboration : 

According  to  Professor  Child's  theories  of  development  and 
reproduction,  any  part  of  a  system,  which,  through  a  lowering 
of  the  rate  of  metabolism  of  the  controlling  part  of  the  system,  say 
the  animal  pole  of  the  blastodermic  vesicle,  is  liable  to  physiologi- 
cal isolation  of  [subordinate']  parts  at  certain  distances  from  the 
dominant  region.  When  such  isolation  of  [subordinate]  parts 
occurs,  new  centers  of  control  arise,  which  produce  outgrowths 
[apical  ends]  capable  of  establishing  new  systems  like  the  original. 

A  familiar  instance  of  agamic  reproduction  in  plants 
will  serve  to  make  this  theory  clear.  In  a  plant  the 
growing  tip  (apical  end)  is  the  dominant  end  of  the 
branch  and  it  seems  to  hold  in  physiological  subordina- 
tion a  considerable  part  of  the  branch  distal  to  itself. 
If,  however,  anything  happens  to  this  growing  tip 
(apical  end)  which  lowers  its  rate  of  metabolism,  a 
whirl  of  secondary  growing  tips  will  appear  just  back 
of  the  original  apical  end,  and  each  of  these  new  apical 
ends  will  become  new  dominant  regions,  capable  of 
producing  new  individuals.^  Any  type  of  environ- 
mental change  that  has  the  effect  of  lowering  the  rate 
of   metabolism   of   the   plant    will  have    the    elTect   of 

^  The  bracketed  words  in  this  quotation  were  not  in  the  original  but 
are  inserted  here  for  the  sake  of  clearness. 

'  In  plants  the  individual  is  not  so  sharply  defined  as  in  most 
animals.  For  our  purposes  we  may  consider  each  growing  point  an 
individual  and  the  whole  plant  a  colony. 


SS  THE  BIOLOGY  OF  TWINS 

inhibiting   the   dominance   of   the   apical   end   and   of 
producing  a  whirl  of  secondary  growing  points. 

Now  in  the  armadillo  egg  the  ectodermic  vesicle  has 
an  apical  point,  which  is  the  head  end  or  growing  tip 
(see  Fig.  12,  X)  of  the  embryo  before  the  process  of 
fission  occurs.  If  the  conditions  of  growth  were  such 
as  to  admit  of  a  normal  rate  of  metabolism,  this  original 
apical  end  would  become  the  head  end  of  a  single 
embryo.  Some  agency  lowers  the  rate  of  metabolism 
of  the  embryo  and  the  original  apical  end  loses  its 
dominance  over  subordinate  regions;  the  result  is  that 
several  radially  arranged  secondary  points  in  the 
ectodermic  vesicle  acquire  independence.  Those  that 
are  most  favorably  situated  with  reference  to  the 
uterine  axes  express  their  independence  first  and  become 
the  first  visible  growing  points,  the  so-called  "primary 
buds";  those  that  are  less  favorably  situated  acquire 
independence  a  little  later  and  form  the  so-called 
''secondary  buds."  It  happens  that,  almost  synchro- 
nously with  the  physiological  isolation  of  the  whirl 
of  subordinate  growing  points,  a  new  and  effective 
nutritive  connection  (the  Trager  ring)  is  established 
between  the  embryonic  vesicle  and  the  maternal  tissues, 
which  greatly  accelerates  the  metabolic  rate  and  the 
consequent  speed  of  growth.  This  rejuvenating  factor 
stops  the  production  of  further  growing  points  and  makes 
it  possible  for  each  of  the  newly  formed  apical  ends 
(heads)  to  develop  a  body.  When  the  conditions  of 
growth  are  restored  to  normal,  the  vesicle  is  no  longer 
a  single  individual  but  is  a  clone,  consisting  of  four 
essentially  separate  individuals,  each  of  which  goes 
through  its   own   embryonic   development   quite  inde- 


THEORIES  OF  POLYEMBRYONIC  DEVELOI'MKXT     89 

pendently  of  the  others,  except  in  so  far  as  devclo])- 
ment  within  a  common  chorion  and  the  necessity  of 
sharing  a  single  primary  placenta  involve  mutual 
adjustments. 

Various  other  theories  purporting  to  olTer  explana- 
tions of  the  production  of  plural  embryos  from  single 
eggs  have  been  advanced,  but  the  three  that  have  been 
most  persistently  maintained  are  the  ''blastotomy 
theory,"  the  "budding  theory,"  and  the  ''fission  theory." 

BLASTOTOMY   VERSUS    BUDDING 

Is  each  embryo  the  lineal  descendant  of  a  single 
blastomere  of  the  four-cell  stage  of  cleavage  or  does 
the  process  of  fission  heretofore  described  occur  with- 
out reference  to  the  cell  products  of  the  early  cleavage 
cells  ?  About  these  contrasting  theories  of  polyem- 
bryony  in  the  armadillo  there  has  been  much  differ- 
ence of  opinion  and  some  shifting  of  viewpoints  on 
the  part  of  individual  workers.  In  one  of  their  earlier 
papers  (1910)  Newman  and  Patterson  stated  that  it 
seems  highly  probable  that  the  tissues  involved  in  each 
of  the  four  quadrants  of  an  embryonic  vesicle  do  really 
arise  as  the  lineal  descendants  of  one  of  the  first  four 
blastomeres.  Still  earlier  in  1909  they  said:  ''In  the 
case  of  Dasypus  each  embryo  probably  arises  from  one 
of  the  blastomeres  of  the  four-celled  stage.''  It  may 
be  said  that,  so  far  as  the  writer  is  concerned,  no  idea 
of  blastotomy  in  the  sense  that  there  was  any  actual 
physical  separation  or  isolation  of  blastomeres  was  ever 
entertained.  Patterson,  however,  in  iqt  ^  in  discussing 
the  various  theories  of  polyembryony,  classes  the 
theory  "that  each  embryo  is  the  Hneal  descendant  of 


go  THE  BIOLOGY  OF  TWINS 

a  single  blastomere  of  the  four-celled  stage"  as  "spon- 
taneous blastotomy."  The  very  meaning  of  this  term 
involves  the  idea  of  physical  separation  of  blastomeres 
and  is  therefore  quite  inappropriate  in  connection  with 
the  idea  that  had  been  expressed  by  the  joint  authors. 
I  quite  agree  that  no  true  "blastotomy"  occurs,  but 
I  would  maintain  that  there  is  much  evidence  for  and 
little  against  the  idea  that  the  cleavage  process  of  the 
armadillo  is  determinate,  that  the  cell  descendants  of 
each  blastomere  of  the  four-cell  stage  constitute 
essentially  a  quadrant  of  the  vesicle  (including  tropho- 
blast,  ectoderm,  endoderm,  etc.),  and  that  therefore  the 
inherited  characters  of  each  embryo  are  dependent  upon 
the  particular  quadrant,  or  parts  of  different  quadrants, 
from  which  it  is  derived. 

Patterson,  however,  totally  abandons  the  idea, 
formerly  entertained  at  least  tacitly  by  him,  that  there 
is  any  connection  between  the  four  embryos  and  the 
four  blastomeres.  This  change  of  opinion  is  doubtless 
due  to  the  observation  that  budding  appears  to  occur 
in  accordance  with  the  bilaterality  of  the  uterus  rather 
than  in  accordance  with  any  lines  of  demarkation 
predetermined  in  the  egg. 

In  a  recent  paper  Wilder  (1916),  in  discussing  the 
armadillo  results,  continues  to  maintain  the  view  that 
there  is  a  real  connection  between  the  four  blastomeres 
and  the  four  fetuses.  He  says  that  it  is  now  clearly 
evident  that  all  ideas  of  physical  separation  of  these 
blastomeres,  a  definite  blastotomy,  does  not  take  place, 
yet  many  things  still  point  to  the  conclusion  that  a 
similar  condition  is  obtained  through  some  form  of 
differentiation,  and  that  each  of  the  separate  embryonal 


THEORIES  OF  POLYEMBRYONIC  DEVELOPMENT     91 

anlages,  be  they  two,  or  four,  eight,  or  eleven  (a  possible 
number  in  Talusia  hybrida^),  is  the  lineal  descendant 
of  a  single  blastomere,  formed  during  early  cleavage. 

THE   FISSION   THEORY 

In  a  paper  read  before  the  Ninth  Zoological  Congress 
at  Monaco,  Assheton  criticizes  both  the  ''blastotomy'' 
and  the  "budding"  theories  of  NewTiian  and  Patterson. 
The  theory  of  budding  seems  to  him  especially  unaccept- 
able. "One  cannot  have  budding,"  he  says,  "unless 
there  is  a  stock  from  which  budding  takes  place.  There 
is  nothing  in  Tatusia  [Dasypus]  one  can  call  a  stock. 
The  phenomenon  is  clearly  that  of  fission." 

In  support  of  the  fission  hypothesis,  he  cites  evidence 
derived  from  the  embryological  study  of  other  mammals, 
notably  the  sheep  and  the  ferret.  "In  the  sheep 
[Ovis]  I  found  some  years  ago  a  blastocyst  at  the  stage 
just  before  the  formation  of  the  embryonal  areas  with 
two  distinct  ectodermic  masses  lying  within  the  tropho- 
blast."  His  outline  figures  show  this  interesting  sheep 
blastocyst  (Figs.  35  and  36)  to  be  covered  by  a  complete 
envelope  of  trophoblast  and  lined  internally  with  a 
complete  layer  of  endoderm.  Such  a  condition  could 
not  have  resulted  from  budding. 

In  the  ferret  {Putorius)  certain  interesting  conditions 
(Figs.  37  and  t^^)  were  found  that  seemed  to  show  evi- 
dences of  a  separation  of  blastomeres,  but  in  no  case 
were  twin  embryos  produced.  These  cases  are  cited 
to  prove  "that  fission  of  the  embryonic  rudiments  of 
eutherian  mammals  may  be  effected  fairly  easily,  but 
the    occurrence   is    the    exception,    not    the    rule."     A 

^  Meaning  Dasypus  hyhridus. 


92 


THE  BIOLOGY  OF  TWINS 


C-bV 


constructive  theory  of  polyembryony  in  Tatusia  (Dasy- 
pus)  is  then  offered,  which  is  based  upon  the  unique 
combination  of  three  conditions: 

(i)  the  development  of  the  blastocyst  within  the  central  lumen 
of  the  uterus  which  has  allowed  of  a  considerable  expansion  of 

the  ectodermic  plate, 
owing  to  the  rolling 
up  of  the  blastocyst 
cavity  as  in  Lupus 
now;  (2)  'inversion  of 
layers'  by  which  the 
ectoderm  plate  be- 
comes invaginated 
into  the  large  cavity 
of  the  blastocyst  sub- 
sequent to  its  ex- 
pansion; (3)  a  late 
formation  of  a  thick- 


FiG.  35 


Fig.  36 


Figs.  35  and  36. — Two  views  of  sheep 
ovum  with  twin  embryo  (after  Assheton). 
It  is  unlikely  that  such  double  embryos 
develop  very  far. 


ened  mass  of  trophoblast  over  the  entire  expanded  plate  putting 
more  pressure  on  the  center  than  on  the  periphery  of  the  ecto- 
dermal disk. 

Assheton  points  out 
that  "if  we  take  a  case 
like  that  of  Lupus  and 
superimpose  upon  it 
the  Trager  of  Mus  we 
should  get  a  condition 
which  would  approxi- 
mately be  that  of 
Tatusia  {DasypusY^ 

This  purely  morphological  explanation  of  what 
seems  to  me  unquestionably  a  physiological  process 
serves  only  to  obscure  the  real  problem  of  the  causal 
basis  of  polyembryony.     I  am,  however,  in  agreement 


W  tmt 


Fig.  37 


Fig.  38 


Figs.  37  and  38. — Two  views  of  a 
double  embryo  of  the  ferret  (after 
Assheton). 


THEORIES  OF  POLYEMBRYONIC  DEVELOPMENT     93 

with  Assheton  in  his  objection  to  the  budchng  hy])othesis 
and  in  considering  the  process  by  which  the  individual 
embryonic  primordia  emancipate  themselves  from  the 
common  germ  as  a  process  oi  fission,  or  dividing  u])  of 
the  ectodermic  vesicle  into  several  ectodermic  primordia, 
each  of  which  stands  for  an  embryonic  area.  The  part 
that  is  left  behind,  the  common  amnion,  is  a  compara- 
tively insignificant  residue  and  could  scarcely  be  termed 
the  stock. 

CONCLUSION 

In  reviewing  all  of  the  facts  it  now  seems  to  me  that 
the  position  that  there  is  a  genetic  connection  between 
the  four  embryos  and  the  four  blastomeres  is  entirely 
untenable.  The  extraordinarily  irregular  arrangement 
and  number  of  fetuses  in  D.  hyhridus  seem  to  be  totally 
out  of  accord  with  this  idea.  The  budding  theory, 
though  affording  a  convenient  descriptive  device, 
appears  to  be  open  to  serious  objection,  as  shown  by 
Assheton,  yet  the  mechanism  of  outgrowth  production  is 
not  unlike  certain  true  cases  of  budding.  The  fission 
idea  seems  to  be  on  the  whole  less  open  to  criticism,  if 
by  fission  we  mean  merely  the  physiological  isolation 
of  several  secondary  points  in  a  single  embryonic 
vesicle,  and  the  consequent  acquisition  by  these  points 
of  independence  in  growth  and  development. 

Unquestionably  long  before  the  isolation  of  several 
secondary  growing  points  a  considerable  amount  of 
differentiation  has  occurred,  so  that  genetic  factors  are 
unequally  distributed  in  the  various  regions  which  give 
rise  to  the  new  apical  points.  We  may  expect  a  less 
degree   of   difference   between   closely   adjacent  points 


94  THE  BIOLOGY  OF  TWINS 

than  between  widely  separated  points;  hence  the 
phenomenon  of  closer  resemblance  between  pairs  derived 
from  one-half  of  the  egg.  Evidently,  too,  as  is  brought 
out  better  in  another  connection,  certain  symmetry 
relations  of  the  entire  egg  are  established  before  the 
formation  of  secondary  growing  points,  and  the  residuum 
of  this  early  symmetric  individuation  is  expressed 
in  mirror-image  resemblances  among  the  quadruplet 
fetuses. 


CHAPTER  V 
TWINNING  IN  RUMINANTS— THE  FREEMARTIN 

All  ruminants  produce  habitually  but  one  offspring 
at  a  birth,  but  in  several  species  (probably  in  all)  two 
or  more  offspring  occasionally  are  born  at  once.  Such 
offspring  are  naturally  spoken  of  as  twins  or  triplets. 
Twinning  in  cattle  and  in  sheep  has  received  consider- 
able attention  because  of  the  economic  aspects  of  the 
case;  if  it  should  prove  to  be  an  inherited  trait,  it 
would  be  a  distinctly  advantageous  character  to  en- 
courage by  breeding. 

Less  attention  has  been  paid  to  twinning  in  wild 
ungulates;  that  the  phenomenon  occurs  in  the  deer  is 
proved  by  the  beautiful  picture  of  a  doe  with  twin  fawns 
published  some  years  ago  in  the  National  Geographic 
Magazine.^  The  twin  fawns  are  remarkably  ahke,  and 
if  one  were  to  judge  by  appearances  alone,  he  might  be 
inclined  to  class  them  as  monozygotic  or  duplicate  twins. 
Such  unsupported  evidence  would,  however,  scarcely 
justify  the  conclusion. 

At  the  present  time  I  have  no  reUable  evidence  of 
twinning  in  horses,  but  it  is  highly  probable  that  this 
group  offers  no  exception  to  the  general  rule  that  all 
mammals  normally  producing  but  a  single  offspring 
at  a  birth  may  have  twins.  There  are  certain  evidences 
of  a  kind  of  twinning  in  swine  involving  the  formation 
of  double  monsters.     That  separate  monozygotic  twins 

^National  Geographic  Magazine,  XXIV  (19 13),  762. 

95 


96  THE  BIOLOGY  OF  TWINS 

occur,  however,  seems  rather  improbable  from  our 
knowledge  of  the  early  embryology  of  swine.  The  field 
needs  further  investigation. 

One  fact  about  the  twins  of  both  cattle  and  sheep  which 
places  them  in  a  dif  event  category  from  armadillo  quad- 
ruplets is  that  the  twins  may  either  both  be  of  the  same  sex 
or  of  opposite  sexes.  If  sex  is  determined  at  the  time  of 
fertilization,  it  seems  unlikely  that  twins  of  opposite 
sexes  could  come  from  one  egg. 

Bovine  twins  may  be  of  four  types:  (a)  two  normal 
males;  {b)  two  normal  females;  (c)  a  normal  male 
co-twin  with  a  normal  female;  {d)  a  normal  male  with 
a  freemartin.  The  exact  nature  of  the  freemartin  is 
a  question  about  which  there  has  been  great  diversity 
of  opinion;  it  is  the  freemartin  situation  which  lends 
special  interest  to  the  study  of  twinning  in  cattle. 

THE   PROBLEM   OF    SEX   IN   THE   FREEMARTIN 

The  freemartin  is  a  sterile  twin  born  co-twin  to  a 
normal  male.  This  definition  is  quite  free  from  implica- 
tion as  to  the  sex  of  the  sterile  individual,  and  advisedly 
excludes  the  so-called  ''fertile  freemartin." 

John  Hunter'  (1786)  appears  to  have  been  the  first 
to  study  the  freemartin  and  to  report  an  opinion  as  to 
its  nature.  He  described  three  specimens  and  diagnosed 
the  conditions  as  follows:  The  first  specimen  (seven 
years  old)  was  apparently  a  hermaphrodite  in  that  it  had 
a  vagina,  a  rudimentary  bicornuate  uterus,  testes,  and 
vasa  deferentia  and  vesiculae  seminales;  the  second 
(five  years  old)  was  "more  like  an  ox  or  spayed 
heifer"  in  general  appearance.     It  had  a  vagina  with 

^  J.  Hunter,  London,  1786. 


TWINNING  IN  RUMINANTS  97 

a  blind  end,  a  two-horned  uterus,  testicles  nearly  as 
large  as  those  of  a  bull,  and  no  ovaries.  Seminal 
vesicles  opened  into  the  vagina,  but  there  were  no  \'asa 
deferentia.  A  clitoris  was  present.  This  animal  was 
preponderatingly  male,  but  showed  organs  of  both  sexes. 
The  third  (three  or  four  years  old)  was  more  like  a  heifer. 
There  was  the  beginning  of  a  vagina  open  as  far  as 
the  urethra,  a  two-horned  uterus,  and  paired  ovaries. 
But  there  were  also  seminal  vesicles  and  part  of  the  vasa 
deferentia.  This  individual  was  predominantly  female, 
but  also  had  organs  of  both  sexes  (a  hermaphrodite). 
Hunter's  view  that  the  freemartin  is  a  transverse 
hermaphrodite  with  a  varying  predominance  of  the  two 
sexes  is  the  classical  one,  and  it  went  unchallenged  for 
some  time. 

The  most  extensive  collection  of  data  up  to  that  of 
Lillie  was  made  by  Numan,'  a  Dutch  investigator,  whose 
paper,  together  with  an  atlas  of  illustrations,  I  have  had 
the  opportunity  of  looking  over.  This  monograph  has 
been  translated  into  French  and  from  the  French  into 
German  by  Spiegelberg.  In  the  course  of  these  transla- 
tions some  accuracy  has  been  lost.  Hart  gives  what 
proves  to  be  a  very  poor  and  inaccurate  translation  of  a 
summary  of  Numan's  findings.  In  brief,  it  may  be  said 
that  Numan  claims  to  have  evidence  of  the  following 
kinds  of  opposite-sexed  twins  in  cattle:  (i)  normal 
male  with  sterile  female  (freemartin) ;  this  is  much  the 
commonest  type;  (2)  normal  male  with  normal  female; 
this  is  a  rare  type;  (3)  normal  female  with  sterile  male; 
this  is  an  extremely  rare  type  and  is  based  on  second- 
hand or  hearsay  information.     Numan  pictures   both 

I  A.  Numan.     Utrecht:   Van  der  Monde,  1843. 


98  THE  BIOLOGY  OF  TWINS 

the  external  and  internal  genitalia  of  a  considerable 
number  of  freemartins  and  shows  clearly  that  they 
range  from  only  shghtly  abnormal  female  types,  in  which 
the  development  of  the  female  organs  is  merely  retarded 
or  juvenile  in  condition,  up  to  those  which  appear  to 
have  developed  certain  positive  male  characters,  such 
as  testes.  Numan's  data  are  extensive  and  valuable, 
but  his  interpretations  are  open  to  question. 

Spiegelberg^  (1861)  was  the  next  investigator  to 
take  up  the  problem  of  the  freemartin.  After  a  careful 
study  of  Hunter's  and  Numan's  works  he  secured  and 
examined  on  his  own  account  two  cases  of  full-term 
twins,  giving  a  detailed  description  of  the  gross  and 
microscopic  anatomy  of  all  significant  parts.  His  con- 
clusion was  that  the  freemartin  is  not  an  imperfect  or 
sterile  female,  but  an  imperfect  male. 

Hart^  gives  a  summary  of  freemartin  literature, 
drawing  largely  from  Numan's  and  Spiegelberg's  data. 
His  own  conclusions  are  totally  erroneous.  He  was 
able  to  make  microscopic  sections  of  the  gonads  of 
Hunter's  freemartins  that  had  been  preserved  in  the 
British  Museum.  In  each  case  they  appeared  to  him 
to  have  the  histological  structure  of  testicular  tissue. 
On  the  basis  of  this  evidence,  taken  with  other  data 
previously  presented,  he  says:  "It  seems  to  me,  there- 
fore, fully  estabHshed  that  the  freemartin,  when  the 
co-twin  of  a  potent  male,  is  a  sterile  male  and  not  a 
sterile  female:  i.e.,  they  are  identical  twins  except  in 
their  genital  tract  and  secondary  sexual  characters." 

^  O.  Spiegelberg,  Ztsch.  fiir  rationalle  Medicin,  Henle  and  Pfeufer, 
Drt.  Reihe,  Bd.  XI  (1861). 

2  D.  Berry  Hart,  Proc.  Royal  Soc.  Edin.,  XXX  (1910). 


TWINNING  IN  RUMINANTS  99 

From  this  and  other  statements  it  is  clear  that  Hart 
considered  the  freemartin  and  its  twin  male  as  deriva- 
tives of  a  single  fertilized  egg  (monozygotic).  On  this 
assumption,  which  is  not  well  founded,  as  we  shall  sec 
later,  he  builds  up  a  hypothesis  to  explain  how  the 
freemartin  arises,  which  may  be  briefly  summed  up  as 
follows:  Thus  the  freemartin  with  a  potent  bull-twin  is 
the  result  of  the  division  of  a  male  zygote,  so  that  the 
somatic  determinants  are  equally  divided,  the  genital 
determinants  unequally  divided,  the  potent  going  to 
the  one  twin,  the  potent  bull,  the  non-potent  genital 
determinants  going  to  the  freemartin.  The  potent 
organs  are  dominant,  the  non-potent  recessive.  The 
Mendelian  scheme  may  be  figured  as  follows: 

Male  sex  gamete     X     Female  sex  gamete 


Male  zygote 

which  if  it  twins 

may  give 


D 


R 


F' 

F" 


Bull  with  equivalent  Bull  with  equivalent 

soma  and  potent  soma  and  non-jwtent  1 

genital  organs  organs  (female  type)  | 

This  MendeUan  view  of  the  freemartin  as  a  pure 
or  extracted  recessive  lacking  its  genital  determinants 
is  one  of  the  oddest  developments  of  the  neo- ■Mendelian 
cult  and  might  be  discussed  pro  and  con  at  some 
length  were  it  not  for  the  fact  that  the  whole  h>pothesis 
is  based  on  an  error,  for  bovine  twins  arc  not  monozygotic. 

It  will  be  noted  that  while  the  earlier  workers  con- 
sidered the  freemartin  as  a  hermaphrodite,  Hart  con- 
siders it  a  recessive  or  non-potent  male,  co-zygotic  with 


lOO  THE  BIOLOGY  OF  TWINS 

a  potent  male.  This  interpretation  is  evidently  due  to 
some  extent  to  the  preconceived  idea  that  the  twins  are 
monozygotic  and  therefore  should  be  of  the  same  sex. 
It  would  seem  unlikely  that  twins  of  opposite  sex  are 
ever  derived  from  a  single  fertilized  egg.  Such  a 
finding  would  be  totally  out  of  accord  with  what  we 
know  of  the  chromosomal  basis  of  sex-determination  in 
mammals.  Yet  such  a  view  has  not  been  without 
adherents.  Bateson,  for  example,  in  his  Problems  of 
Genetics  makes  the  following  statement  about  free- 
martins  : 

In  horned  cattle  twin  births  are  rare,  and  when  types  of 
twins  of  opposite  sexes  are  born,  the  male  is  perfect  and  normal, 
but  the  reproductive  organs  of  the  female  [italics  mine]  are 
deformed  and  sterile,  being  known  as  a  freemartin.  The  same 
thing  occasionally  occurs  in  sheep,  suggesting  that  in  sheep  also 
twins  may  be  formed  by  the  division  of  one  ovum  [italics  mine]; 
for  it  is  impossible  to  suppose  that  mere  development  in  juxta- 
position can  produce  a  change  of  this  character.  I  mention  the 
freemartin  because  it  raises  a  question  of  absorbing  interest.  It 
is  conceivable  that  we  should  interpret  it  by  reference  to  the 
phenomenon  of  gynandromorphism,  seen  occasionally  in  insects, 
and  also  in  birds  as  a  great  rarity.  In  the  gynandromorph  one 
side  of  the  body  is  male,  the  other  female.  In  such  cases  neither 
side  is  sexually  perfect.  If  the  halves  of  such  a  gynandromorph 
came  apart,  perhaps  one  would  be  a  freemartin. 

This  statement  commits  Bateson  to  the  theory  of 
monozygotic  origin  of  heterosexual  cattle  and  sheep 
twins  and  to  the  interpretation  of  the  freemartin  as 
a  sterile  female. 

Recently  Cole^  in  a  brief  abstract  takes  a  view  of 
the  value  of  the  freemartin  quite  in  accord  with  that 

^  L.  J.  Cole,  Science,  N.S.,  XLIII  (1916). 


TWINNING  IN  RUMINANTS  loi 

of  Hart,  and  cites  certain  statistical  evidence  in  fa\or  of 
it.     As  the  abstract  is  so  brief  it  may  be  quoted  in  full: 

A  study  of  303  multiple  births  in  cattle,  obtained  directly 
from  the  breeders.  The  records  include:  43  cases  homosexual 
male,  165  cases  recorded  heterosexual  (male  and  female),  88  cases 
homosexual  female,  7  cases  triplets,  a  ratio  of  twins  approx- 
imately 1:4:2  instead  of  i  :  2  :  i  expected,  if  there  were 
no  disturbing  element  entering  in.  The  expectation  may  be 
brought  more  nearly  into  harmony  with  the  facts  if  it  is  assumed 
that  in  addition  to  ordinary  fraternal  (dizygotic)  twins  there  are 
numbers  of  "identical"  (monozygotic)  twins  of  both  sexes,  and 
that  while  in  the  case  of  females  those  are  both  normal,  in  the 
cases  of  a  dividing  male  zygote,  to  form  two  individuals,  in  one 
of  them  the  sexual  organs  remain  in  the  undifferentiated  stage, 
so  that  the  animal  superficially  resembles  a  female  and  is  ordi- 
narily recorded  as  such,  although  it  is  barren.  The  records  of 
monozygotic  twins  accordingly  go  to  increase  the  homosexual 
female  and  the  heterosexual  classes,  while  the  homosexual  male 
class,  in  which  part  of  them  really  belong,  does  not  receive  any 
increment.  This  brings  the  expected  ratio  much  nearer  the 
ratio  obtained. 

Any  female  calf  twinned  with  a  male  is  referred  to  as  a 
freemartin.  According  to  the  interpretation  given,  some  free- 
martins  should  be  fertile  while  others  are  sterile.  It  was  found 
that  both  exist. 

It  will  be  noted  that  Cole  interprets  the  freemartin 
as  an  undeveloped  male  in  which  the  sex-organs  remain 
in  the  undifferentiated  state  and  thus  resemble  those 
of  a  juvenile  female.  This  view  of  the  freemartin  as 
a  male  is  a  concession  to  the  idea  that  monozygotic 
twins  should  be  of  the  same  sex,  since  sex  is  supposed 
to  be  determined  at  the  time  of  fertilization. 

As  is  always  the  case  when  expectations  are 
based  on  incomplete  data,   these  numerous  divergent 


I02  THE  BIOLOGY  OF  TWINS 

interpretations  as  to  the  nature  and  mode  of  origin  of 
the  freemartin  are  all  quite  mistaken;  now  that  we  have 
the  problem  really  cleared  up  they  seem  almost  equally 
absurd.  The  Mendelian  interpretation  of  Hart,  the 
suggestion  involving  g^mandromorphism  of  Bateson, 
and  the  inferences  of  Cole  based  on  sex-ratios  appear 
alike  far-fetched  in  the  hght  of  further  facts  and  more 
reliable  conclusions. 

Recently  F.  R.  LilHe'  has  solved  the  mystery  of  the 
freemartin  through  an  embryological  study  of  twinning 
in  cattle.  The  material  has  been  collected  during  the 
past  two  or  three  years,  and  I  have  been  much  interested 
in  the  progress  of  the  research.  Lilhe's  preliminary 
paper  was  called  forth  as  a  reply  to  Cole's  report  just 
given;  he  criticizes  Cole's  data  and  his  interpretations, 
and  says: 

I  wish  to  point  out  the  fatal  objection  that,  according  to  the 
hj^Dothesis,  the  females  remaining  in  the  heterosexual  class  are 
normal;  in  other  words,  on  this  hypothesis,  the  ratio  of  normal 
freemartins  (females  co-twin  with  a  bull)  to  sterile  ones  is  3  :  i; 
and  the  ratio  would  not  be  very  different  on  any  basis  of 
division  of  the  heterosexual  class  that  would  help  out  the 
sex-ratio.  Hitherto  there  have  been  no  data  from  which  the 
ratio  of  normal  to  sterile  freemartins  could  be  computed,  and 
Cole  furnishes  none.  I  have  records  of  21  cases  statistically 
homogeneous,  three  of  which  are  normal  and  18  abnormal.  That 
is,  the  ratio  of  normal  to  sterile  freemartins  is  i   :   6  instead  of 

3:1- 

This  ratio  is  not  more  adverse  to  the  normals  than  might  be 

anticipated,  for  breeders'  associations  will  not  register  freemartins 

until  they  have  proved  capable  of  breeding,  and  some  breeders 

hardly  believe  in  the  existence  of  fertile  freemartins,  so  rare  are 

they. 

^F.  R.  Lillie,  Science,  N.S.,  XLIII  (1916). 


TWINNING  IN  RUMINANTS  103 

Having  shown  the  untenability  of  Cole's  con- 
clusions, Lillie  presents  the  results  of  his  own  examina- 
tion of  41  cases  of  bovine  twins,  all  examined  in  utcro. 
The  material  was  collected  from  the  Chica<^()  stuekvards 
by  a  skilled  assistant,  and  in  every  case  the  ovaries  of 
the  mother  were  obtained  and  the  embryonic  envelopes 
of  the  fetuses  were  preserved  and  examined.  The 
nature  of  the  fetal  genitaha  was  determined  by  dissec- 
tion, and  in  some  cases  by  sectioning.  No  such  body 
of  data  has  previously  been  obtained  on  bovine  twins. 
This  work  is  still  in  progress  and  will  be  reported  at 
length  in  due  time.  In  the  meantime  it  will  be  well  to 
give  here  nearly  all  of  the  data  furnished  by  Lillie 's 
abstract  in  Science. 

Out  of  41  cases  of  bovine  twins  14  are  both  male, 
21  are  of  opposite  sexes,  and  6  are  both  female.  This 
is  about  what  one  would  expect,  if  we  interpret  the  free- 
martin  as  a  female,  for  there  are  about  as  many  same- 
sexed  twins  as  opposite-sexed.  The  number  of  the 
same-sexed  male  twins  is  higher  than  expected,  but 
perhaps  this  is  due  to  the  comparatively  small  number 
of  cases.  If,  according  to  Hart  and  Cole,  frecmartins 
also  are  males,  there  would  be  an  enormous  and  inexpli- 
cable preponderance  of  males.     As  Lilhe  says: 

The  real  test  of  the  theory  must  come  from  the  embr>'ological 
side.  If  the  sterile  freemartin  and  its  bull-mate  are  monozygotic, 
they  should  be  included  in  a  single  chorion,  and  there  should  be 
a  single  corpus  luteum  present.  If  they  are  dizygotic,  wc  might 
expect  two  separate  chorions  and  two  corpora  lutea.  'ihe 
monochorial  condition  would  not,  however,  be  a  conclusive  test 
of  monozygotic  origin,  for  two  chorions,  originally  independent, 
might  fuse  secondarily.  The  facts  as  determined  from  examina- 
tion of  41  cases  are  that  about  97.5  per  cent  of  bovine  twins  are 


I04  THE  BIOLOGY  OF  TWINS 

monochorial,  but  in  spite  of  this  nearly  all  are  dizygotic  [italics 
mine] ;  for  in  all  cases  in  which  the  ovaries  were  present  with  the 
uterus  a  corpus  liiteum  was  present  in  each  ovary  [italics  mine]; 
in  normal  single  pregnancies  in  cattle  there  is  never  more  than 
one  corpus  luteum  present.  There  was  one  homosexual  case 
(males)  in  which  only  one  ovary  was  present  with  the  uterus 
when  received,  and  it  contained  no  corpus  luteum.  This  was 
probably  monozygotic. 

In  cattle  a  twin  pregnancy  is  almost  always  the  result  of  the 
fertilization  of  an  ovum  from  each  ovary;  the  development 
begins  separately  in  each  horn  of  the  uterus.  The  rapidly  elongat- 
ing ova  meet  and  fuse  in  the  small  body  of  the  uterus  at  the 
same  time  between  the  lo  mm.  and  20  mm.  stage.  The  blood 
vessels  from  each  side  then  anastomose  in  the  connecting  part 
of  the  chorion;  a  particularly  wide  anastomosis  develops,  so  that 
either  fetus  can  be  injected  from  the  other.  The  arterial  circula- 
tion of  each  overlaps  the  venous  territory  of  the  other,  so  that 
constant  interchange  of  blood  takes  place  [Fig.  40].  If  both  are 
males  or  both  are  females,  no  harm  results  from  this;  hut  if  one 
is  male  and  the  other  female,  the  reproductive  system  of  the  female 
is  largely  suppressed,  and  certain  male  organs  even  develop  in  the 
female.  This  is  unquestionably  to  be  interpreted  as  a  case  of 
hormone  action.  It  is  not  yet  determined  whether  the  invariable 
result  of  sterilization  of  the  female  at  the  expense  of  the  male  is 
due  to  more  precocious  development  of  the  male  hormones,  or 
to  a  certain  natural  dominance  of  male  over  female  hormones. 

The  results  are  analogous  to  Steinach's  feminization  of  male 
rats  and  masculinization  of  female  by  heterosexual  transplanta- 
tion of  gonads  into  castrated  infantile  specimens.  But  they  are 
more  extensive  in  many  respects  because  of  the  incomparably 
earlier  onset  of  the  hormone  action.  In  the  case  of  the  freemartin, 
nature  has  performed  an  experiment  of  surpassing  interest. 

Bateson  states  that  sterile  freemartins  are  found  also  in 
sheep,  but  rarely.  In  the  four  twin  pregnancies  of  sheep  that  I 
have  so  far  had  the  opportunity  to  examine,  a  monochorial 
condition  was  found,  though  the  fetuses  were  dizygotic;  but  the 
circulation  of  each  fetus  was  closed.  This  appears  to  be  the 
normal  condition  in  sheep;    but  if  the  two  circulations  should 


TWINNING  IN  RUMINANTS  105 

anastomose,  we  should  have  the  conditions  that  produce  a  sterile 
freemartin  in  cattle.  The  possibility  of  their  occurrence  in  sheep 
is  therefore  given. 

The  fertile  freemartin  in  cattle  may  be  due  to  causes  similar 
to  those  normal  for  sheep.  Unfortunately,  when  the  first  two 
cases  of  normal  cattle  freemartins  that  I  have  recorded  came 
under  observation,  I  was  not  yet  aware  of  the  significance  of  the 
membrane  relations,  and  the  circulation  was  not  studied.  But  I 
have  in  my  notebook  in  each  case  that  the  connecting  link  of  the 
two  halves  of  the  chorion  was  narrow,  and  this  is  significant. 
In  the  third  case  the  two  chorions  were  entirely  unfuscd;  this 
constitutes  an  experimentum  crucis.  The  male  was  10.4  cm. 
long;  the  female,  10.2  cm.  The  reproductive  organs  of  both 
were  entirely  normal.  The  occurrence  of  the  fertile  freemartin  is 
therefore  satisfactorily  explained. 

The  sterile  freemartin  enables  us  to  distinguish  between  the 
effect  of  the  zygotic  sex-determining  factor  in  mammals  and  the 
hormone  sex-differentiating  factors.  The  female  is  sterilized 
at  the  very  beginning  of  sex-differentiation,  or  before  any  morpho- 
logical evidences  are  apparent,  and  the  male  hormones  circulate 
in  its  blood  for  a  long  period  thereafter.  But  in  spite  of  this,  the 
reproductive  system  is,  for  the  most  part,  of  the  female  t>'pe, 
though  greatly  reduced.  The  gonad  is  the  part  most  afi"ected; 
so  much  so,  that  most  authors  have  interpreted  it  as  a  testis;  a 
gubernaculum  of  the  male  type  also  develops,  but  no  scrotal  sacs. 
The  ducts  are  distinctly  of  the  female  type  much  reduced,  and 
the  phallus  and  mammary  glands  are  definitely  female.  The 
genital  somatic  habitus  inclines  toward  the  male  side.  Male 
hormones  circulating  in  the  blood  of  an  individual  zygotically 
female  have  a  definitely  limited  influence  even  though  the  action 
exists  from  the  beginning  of  morphological  sex-dilTcrentiation. 

The  drawing  of  twin  calves  shown  in  Fig.  39  shows 
very  clearly  the  intra-uterine  relations  of  bovine  twins. 
For  permission  to  use  this  figure  I  am  much  indebted  to 
Professor  Lillie,  whose  monograph  on  cattle  twins  is 
now  in  course  of  preparation.     The  figure  shows  twins 


io6 


THE  BIOLOGY  OF  TWINS 


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TWINNING  IN  RUMINANTS  107 

about  ten  inches  long  taken  out  of  the  chorionic  sac 
through  the  openings  shown  in  dark  shading.  The 
double  chorionic  vesicle  is  shown  somewhat  collapsed 
through  loss  of  its  fluid  contents.  The  narrow  part 
in  the  middle  is  the  point  where  two  separate  eggs  have 
fused  at  a  much  earlier  period.  The  flower-like  pads 
scattered  over  the  membranous  wall  are  the  placental 
areas,  the  so-called  cotyledons,  by  means  of  which  the 
fetuses  obtain  nourishment  from  the  uterine  tissues. 
The  chorionic  blood  vessels  and  those  of  the  fetuses  are 
shown  clearly,  the  arteries  in  outline  and  the  veins 
black.  It  will  be  seen  that  the  arteries  of  the  two 
fetuses  are  in  direct  communication  across  the  placental 
bridge.  The  veins  of  both  fetuses  in  some  cases  enter 
the  same  cotyledon.  There  is  every  opportunity  for 
the  admixture  of  blood  between  the  twin  individuals. 
It  need  hardly  be  pointed  out  that  the  individual  on 
the  left  is  a  male  and  that  on  the  right  a  sterile  female 
or  freemartin. 

LilHe's  work  has  revealed  the  true  nature  of  the 
freemartin;  it  is  a  sterile  female  whose  gonads  remain 
in  the  juvenile  stage  so  that  they  resemble  testes,  and 
which  has  certain  secondary  sexual  characters  of  the  male 
due  to  the  presence  for  a  considerable  period  of  male 
hormones  in  the  blood  borrowed  from  its  male  co-twin. 
The  animal  is  a  hermaphrodite  only  in  a  very  limited 
sense.  The  work  leaves  no  question  as  to  the  dizygotic 
origin,  not  only  of  opposite-sexed,  but  also  of  same- 
sexed  bovine  twins. 

Whether  real  monozygotic  twinning  ever  occurs 
among  the  ungulates  is  highly  questionable.  Several 
considerations  lead  to  this  conclusion.     Polyembryony, 


io8  THE  BIOLOGY  OF  TWINS 

in  those  mammals  that  unquestionably  show  it,  has  a 
definite  relation  to  certain  pecuHar  types  of  simple  uteri 
and  a  special  type  of  embryonic  development  called 
germ-layer  inversion.  In  the  armadillo  it  is  difficult  to 
imagine  how,  apart  from  germ-layer  inversion,  the 
complete  separation  of  embryos  could  occur  and  the 
monochorial  condition  be  still  maintained.  In  man 
the  conditions  described  for  the  Bryce  and  Teacher 
ovum  appear  to  indicate  a  situation  like  that  of  the 
armadillo;  but  no  such  condition  has  been  described  for 
any  of  the  ungulates.  It  is  barely  possible  that  blas- 
tomeres  could  separate  and  form  two  individual  embryos, 
but  this  would  involve  a  subsequent  fusion  of  chorions  just 
like  that  which  occurs  in  the  dizygotic  twins  of  Euphrac- 
tus  villosus.  Lillie  cited  one  equivocal  case  of  apparent 
monozygotic  twins.  In  one  pair  of  twin  males  the 
genitalia  of  the  mother  were  roughly  handled  so  that 
when  examined  but  one  ovary  was  present.  This 
ovary  had  no  corpus  luteum.  The  inference  made  is 
that  the  lost  ovary  had  but  one  corpus  luteum,  since 
in  all  other  cases  examined  but  one  corpus  luteum 
occurs  to  an  ovary.  It  seems  probable,  however,  that 
the  lost  ovary  would  have  shown  two  corpora  lutea 
had  it  been  examined.  That  an  ovary  may  ovulate 
two  ova  at  a  time  is  seen  from  the  fact  that  triplets, 
which  may  be  either  of  the  same  sex  or  of  opposite  sexes, 
not  infrequently  occur.  That  calf  twins  may  be 
monozygotic  is  believed  by  several  writers.  Pearl, 
for  example,  in  a  paper  on  '' Triplet  Calves"  states 
that  the  two  nearly  identical  females  (see  two  outer 
calves  in  Fig.  40)  are  possibly,  if  not  probably,  derived 
from  the  first  two  blastomeres  of  a  single  dividing  egg. 


TWINNING  IN  RUMINANTS 


109 


This  condition  seems  to  me  barely  possible,  but  very 
improbable. 

Twinning  in  the  genus  Dasypiis  and  twinning  in 
ruminants  turn  out  to  be  two  totally  different  phenom- 
ena. In  Dasypus,  the  twins  are  monozygotic  and 
always  of  the  same  sex.     In  cattle,  twins  are  dizygotic 


Fig.  40. — Photograph  of  triplet  calves  (from  Peari) 

I  and  may  or  may  not  be  of  the  same  sex.  The  additional 
phenomenon  of  freemartinism  adds  materially  to  the 
value  of  bovine  twins,  especially  in  its  bearings  on  the 
problems  of  sex-biology.  In  a  subsequent  chapter 
some  of  these  bearings  receive  a  more  adequate  dis- 
cussion in  connection  with  other  sex-phenomena  in 
twins  than  is  possible  at  this  time. 


CHAPTER  VI 

TWINS  IN  RELATION  TO  GENERAL  BIOLOGICAL 

PROBLEMS 

The  study  of  twins  throws  light  especially  on  the 
following  problems:  (i)  the  time  of  and  the  mechanics 
of  sex-determination;  (2)  the  significance  of  sex-ratios; 
(3)  the  mechanism  of  sex-differentiation;  (4)  is  twin- 
ning hereditary?  (5)  the  modes  of  inheritance  in 
monozygotic  or  polyembryonic  twins;  (6)  the  nature 
and  significance  of  symmetry  reversals  in  monozygotic 
twins.  The  first  five  problems  are  discussed  in  the 
present  chapter,  while  the  last  two  problems  are  reserved 
for  the  next  chapter. 

TWINS    AND    THE    PROBLEM    OF    SEX-DETERMINATION 

The  problems  of  sex  are  today  attracting  the  widest 
attention,  and  among  these  problems  that  of  the  mechan- 
ism of  sex-determination  appears  to  have  been  largely 
solved.  It  appears  that  in  a  vast  number  of  animals 
of  all  grades  of  organization,  from  worms  to  man,  sex 
is  determined  at  the  time  of  fertilization.  In  some 
forms  sex  is  determined  in  the  egg,  for  there  are  two 
distinct  types  of  eggs,  male-producing  and  female- 
producing.  In  other  cases  the  eggs  are  all  alike  and 
produce  females  if  allowed  to  develop  partheno- 
genetically  (without  fertilization),  but  produce  half 
males  and  half  females  if  fertilized,  the  result  being  due 

no 


RELATION  TO  GENERAL  BIOLOGICAL  PROBLEMS    iil 

to  two  kinds  of  spermatozoa,  male-producing  and 
female-producing.  In  still  other  cases,  notably  the 
hymenoptera,  males  are  produced  when  the  eggs  are  not 
fertilized  and  females  when  the  eggs  are  fertilized.  All 
of  these  apparently  divergent  phenomena  are  consistent 
with  the  idea  that  sex  is  determined  in  the  germ-cell 
and  that  the  sex-determining  factor  is  in  some  way 
intimately  associated  with  the  presence  of  a  peculiar 
chromosome  (the  X  chromosome),  or  group  of  chromo- 
somes, in  the  nucleus  of  the  germ-cell.  This  mechanism 
gives  a  sex-bias  to  the  individual,  a  bias  in  some  cases 
so  strong  that  no  known  factors  can  interfere  with  the 
fulfilment  of  the  sex-development  that  was  originally 
determined.  In  other  cases,  however,  sex  may  be 
zygotically  determined,  but  requires  a  definite  favorable 
environment  to  bring  it  to  complete  development  or 
differentiation.  Finally,  in  some  cases  the  individual 
which  is  zygotically  sex-determined  may  have  its 
sex-development  so  altered  as  to  become  largely  of  the 
opposite  sex. 

In  mammals  there  is  much  evidence  that  sex  is 
zygotically  determined;  there  appear  to  be  two  kinds 
of  spermatozoa  and  but  one  kind  of  egg,  and  the  sex  of 
the  individual  depends  on  whether  a  male-producing  or  a 
female-producing  spermatozoon  fertilizes  a  particular  egg. 

If  then  sex  in  mammals  is  determined  in  the  unde- 
veloped egg,  we  should  naturally  expect  twins  or  large 
numbers  of  individuals  derived  from  a  single  egg  to  be 
of  the  same  sex.  This  is  just  the  point  upon  which 
monozygotic  twinning  and  pol}embryony  ha\e  a 
bearing,  for  in  these  phenomena  we  have  experiments 
demonstrating  the  theory  of  zygotic  sex-determination. 


1 1 2  THE  BIOLOGY  OF  TWINS 

If  we  were  able  artificially  to  subdivide  a  fertilized 
egg  into  two  or  more  parts  and  the  individuals  develop- 
ing from  the  parts  of  a  given  egg  were  always  of  the 
same  sex,  we  should  consider  the  theory  of  zygotic  sex- 
determination  as  proven.  Our  nicest  technique,  how- 
ever, has  not  been  adequate  to  carry  out  so  crucial 
an  experiment,  and  we  are  therefore  forced  to  rely 
upon  an  equivalent  experiment  in  nature.  It  has  been 
shown  conclusively  for  two  species  of  armadillo,  and  by 
analog>^  for  man,  that  an  egg,  divided  at  an  early 
period  into  two  or  more  embryonic  primordia,  produces 
individuals  all  of  the  same  sex.  In  hundreds  of  sets 
of  quadruplets  in  the  Texas  armadillo  there  has  occurred 
no  exception  to  this  rule,  in  spite  of  the  fact  that  in  some 
cases  there  are  marked  differences  in  size  due  to  unequal 
environment  factors.  In  one  case  two  fetuses  of  a  set 
are  nearly  twice  the  size  of  two  others,  yet  the  sex  of  all 
is  the  same,  showing  that  the  zygotically  determined 
sex  is  incapable  of  alteration  through  any  ordinary 
environmental  change.  Similarly,  in  man  twins  that 
are  monochorial  and  in  other  ways  bear  evidences  of 
monozygotic  origin  are  always  of  the  same  sex.  Con- 
joined twins,  which  are  unquestionably  monozygotic, 
are  also  same-sexed,  although  one  half  of  an  individual 
may  be  much  better  developed  than  the  other. 

Not  only  in  mammals  but  also  in  other  animals 
exhibiting  polyembryony  it  is  true  that  all  individuals 
derived  from  a  single  germ-cell  are  same-sexed;  several 
investigators  have  shown  this  for  various  genera  of 
parasitic  hymenoptera.  Silvestri  was  the  first  author 
to  discover  polyembryony  in  these  animals.  He  found 
that  in  the  genus  Litomastix,  in  which  the  eggs  are  laid 


RELATION  TO  GENERAL  BIOLOGICAL  PROBLEMS    113 

in  the  body  of  a  caterpillar,  a  single  egg  divides  very 
early  into  a  very  large  number  of  separate  embryonic 
primordia,  each  of  which  produces  an  adult  insect. 
No  matter  how  many  individuals  are  derived  from  one 
egg — the  number  may  be  even  a  thousand — they  are  of 
the  same  sex.  Some  get  more  food  than  others,  grow 
faster,  and  become  larger,  but  nothing  environmental 
affects  the  zygotically  determined  sex.  The  mode  of  zy- 
gotic sex-determination  is  somewhat  peculiar  in  that 
eggs  develop  whether  fertilized  or  not;  if  fertilized  all 
offspring  from  that  egg  are  females;  if  not  fertilized, 
but  developing  parthenogenetically,  all  offspring  from 
a  given  egg  are  males. 

If  then  in  two  groups  as  far  apart  as  mammals  and 
hymenoptera  the  fact  of  zygotic  sex-determination  is 
proved  by  the  phenomenon  of  polyembryony,  it  seems 
probable  that  this  is  a  very  general  principle;  possibly 
universal. 

SEX-RATIOS    IN   TWINS   AND   MULTIPLE   BIRTHS 

Certain  facts  derived  from  a  statistical  study  of  the 
sexes  of  twins  of  various  species  seem  at  first  sight  to  be 
opposed  to  the  theory  of  rigid  zygotic  sex-determination 
in  mammals.  It  would  appear  that  the  proportion  of 
males  to  females  in  twins  and  multiple  offspring,  whether 
monozygotic  or  dizygotic,  should  not  differ  from  that 
which  holds  for  the  species  in  general.  In  mammals 
the  ratio  of  males  to  females  is  in  general  quite  close 
to  I  :  I,  although  a  sHght  preponderance  of  males  is 
usually  present. 

In  man,  for  example,  it  has  been  shown  by  several 
writers  that  as  the  number  of  individuals  to  a  birth 


114  THE  BIOLOGY  OF  TWINS 

increases  the  relative  proportion  of  males  to  females 
decreases.     Nichols  {loc.  cit.)  gives  the  following  table: 

Number  of  Sons  for 
i,ooo  Daughters 

Single  births 1,057 

Twin  births 1,043 

Triple  births 1,007 

Quadruple  births 548 

Similar  ratios  are  found  in  sheep.  The  following 
table  is  attributed  by  Wentworth^  to  Pearl.  In  115 
multiple  sets  in  sheep  the  following  sex-ratios  occurred: 

3  males  to  a  birth ■ 16 

2  males  and  i  female  to  a  birth 39 

2  females  and  i  male  to  a  birth 22 

3  females  to  a  birth 38 

A  summary  shows  that  the  ratio  of  females  to  males  in 
these  large  plural  births  is  197  females  to  148  males. 

The  reason  for  the  preponderance  of  females  in 
the  plural  offspring  of  these  two  species  is  not  fully 
known;  the  basis  of  it  is  probably  a  sex-difference  in 
prenatal  mortality.  It  is  well  known  that  the  prenatal 
mortality  of  human  male  twins  is  greater  than  that  of 
females,  an  indication  that  males  are  less  resistant  to 
abnormal  uterine  conditions  than  are  females.  In  the 
uterus  of  a  species  adapted  for  uniparous  gestation  the 
crowding  of  several  fetuses  must  inevitably  introduce 
untoward  conditions;  if  males  are  more  susceptible 
to  subnormal  conditions  than  females  it  is  highly 
probable  that  they  succumb  in  larger  numbers  than  the 
latter. 

That  the  mere  fact  of  multiple  offspring  carries 
with  it  no  necessary  disturbance  of  sex-ratios  is  seen 

^E.  N.  Wentworth,  Science,  N.  S.,  XXXIX  (1914). 


RELATION  TO  GENERAL  BIOLOGIC.\L  PROBLEMS    115 


when  the  ratios  of  normally  multiparous  species  are 
examined.  Wentworth  gives  the  following  tables  (I 
and  II)  for  swine  and  for  dogs: 


TABLE   I 

Sex-Ratios  in  Swine  Births 

No.  of  males  per  litter  .... 
Expectation  (in  no.  of 

litters)  

Actual  no.  of  litters 

.0 

3-4 

2 

I 

12.8 
13 

2 

24.  2 
26 

3 

33-4 
28 

4 

34-7 
31 

5 

28. s 
28 

No.  of  males  per  litter  .... 
Expectation  (in  no.  of 

litters) 

Actual  no.  of  litters 

6 

19 

21 

7 

10.4 
12 

8 

4.6 
8 

9 

1-7 
2 

10 

0.48 
2 

II 

0.  II 

I 

TABLE   II 
Sex-Ratios  in  Dog  Litters 


No.  of  male  pups  per  litter 
Expectation  (in  no.  of 

litters) 

Actual  no.  of  litters 


0 

I 

2 

3 

4 

5 

151 

35-75 

37 

245 

10.86 

2.8 

14 

36 

39 

22 

II 

4 

■3 


In  the  total  of  173  litters  of  swine  there  is  really  no 
significant  departure  from  the  normal  distribution  of 
the  sexes. 

Again  no  disturbance  of  the  expected  ratio  occurs 
in  a  total  of  126  litters  of  pups. 

From  these  statistics  it  appears  that  the  disturb- 
ances in  sex-ratios  of  plural  births  are  limited  to  those 
species  that  are  normally  uniparous.  This  may  be  due 
partly  to  differential  prenatal  mortality,  favoring  the 
survival  of  females.  There  is  nothing  about  these 
ratios  at  all  out  of  accord  with  the  idea  that  in  mammals 
sex  is  zygotically  determined. 


Ii6  THE  BIOLOGY  OF  TWINS 

SEX-DIFFERENTIATION   IN   MAMMALS 

Although  sex  is  zygotically  determined  in  mammals, 
the  differentiation  of  sex-characters  depends  on  a 
secondary  mechanism  that  is  believed  to  be  associated 
with  an  internal  secretion  of  the  gonads.  It  has  been 
long  known  that  castration  or  ovariotomy  of  young 
mammals  prevents  the  development  of  adult,  sexual 
characters  and  the  individual  remains  a  neuter,  though 
zygotically  either  a  male  or  a  female.  Steinach  in  a 
brilliant  series  of  experiments  with  rats  has  shown  that  a 
transplantation  of  ovaries  into  a  young  castrated  male 
markedly  alters  the  sex-differentiation  of  the  operated 
individual,  making  it  take  on  many  of  the  characters 
of  the  female;  even  milk  glands  become  functional 
and  a  maternal  instinct  develops.  Conversely,  a  transfer 
of  the  testicular  tissue  into  an  ovariotomized  female 
tends  to  masculinize  the  animal,  so  that  it  becomes  large 
like  the  male  and  exhibits  the  pugnacious  character 
of  the  latter.  Since  only  the  glandular  portion  of  the 
transplanted  ovary  or  testis  survives  in  these  experi- 
ments there  is  no  alternative  but  to  attribute  the  reversal 
of  sex- tendency  to  some  secretion  of  these  glands,  and, 
for  a  lack  of  a  better  term,  the  active  principle  has  been 
called  hormone.  The  interstitial  glandular  tissue  of  the 
testis  secretes,  therefore,  male-differentiating  hormones 
and  the  equivalent  tissue  of  the  ovary  secretes  female- 
differentiating  hormones.  These  hormones  must  be 
given  off  into  the  blood,  for  it  affects  all  parts  of  the 
body. 

Crucial  as  are  the  experiments  in  transplantation  of 
gonads,  they  do  not  equal  in  subtlety  and  finish  the 
experiment  of  Nature  performed  in  the  case  of  the  free- 


RELATION  TO  GENERAL  BIOLOGICAL  PROBLEMS    117 


martin.  Here  an  individual  zygotically  determined 
to  be  a  female  twin  may  become  more  or  less  com- 
pletely differentiated  into  a  male  by  the  very  neat 
device  of  borrowing  hormone-charged  blood  from  its 
male  co-twin. 

If  two  male  twins  mutually  transfuse  blood,  no 
alteration  of  the  sex-bias  occurs.  The  same  is  true 
in  the  case  of  two  female  twins.  But  wherever  the 
twfns  are  opposite-sexed,  the  female  is  the  one  to  suffer 
sex-reversal.  At  first  it  was  not  clear  to  Lillie  whether 
this  result  was  due  to  a  dominance  of  male-differentiating 
hormones  over  female,  or  was  the  result  of  a  precocious 
development  of  male  glands.  Further  studies  favor  the 
latter  interpretation,  and  it  now  appears  that  the  glan- 
dular portion  of  the  testis  differentiates  before  that 
of  the  ovary  and  that  w^hen  the  twins  unite  blood 
vessels  in  the  chorion,  the  blood  of  the  male  passes  into 
the  system  of  the  female  and  inhibits  the  development 
of  the  glandular  portion  of  the  ovary.  In  the  absence 
of  female-differentiating  hormones  in  the  zygotically 
determined  female,  the  male  hormones  have  full  play 
and  actually  do  cause  the  differentiation  of  male  charac- 
ters in  the  freemartin.  The  freemartin  is  then  a  paradox : 
it  is  a  zygotic  female,  but  differentiated  more  or  less 
extensively  into  a  male. 

These  results  make  it  necessary  then  to  distinguish 
most  carefully  between  sex-determination  and  sex- 
differentiation.  Sex  may  be  determined  zygotically, 
but  may  be  altered  more  or  less  completely  by  a  change 
in  the  hormones.  The  chromosome  mechanism  appears 
to  fix  the  sex-bias  and  the  hormone  mechanism  to 
bring    about    sex-differentiation.     Normally,    however, 


Ii8  THE  BIOLOGY  OF  TWINS 

the  zygotically  determined  sex  remains  unaltered,  for  a 
zygotic  female  will  produce  female-differentiating  hor- 
mones and  will  produce  an  adult  female  individual. 

IS    TWINNING  HEREDITARY? 

In  the  polyembryonic  armadillos  (Dasypus  novem- 
cinctus  and  D.  hyhridus)  it  goes  without  saying  that 
their  peculiar  process  of  twinning  is  hereditary ;  it  is  the 
only  mode  of  reproduction  in  these  species.  What  is 
really  inherited  in  these  cases  is  not  fully  understood, 
but  it  is  believed  that  its  basis  lies  in  some  physiological 
peculiarity  of  the  egg,  which  causes  it  to  have  an  abnor- 
mally slow  early  development.  As  brought  out  in  the 
earlier  chapter,  this  retardation  in  the  developmental 
rhythm  produces  an  early  fission  of  the  embryonic 
materials  into  several  distinct  primordia  each  of  which 
produces  an  embryo.  Whatever  the  cause  of  poly- 
embryony,  it  is  unquestionably  a  specific  character  and 
therefore  hereditary. 

In  the  armadillo  Euphractus  villosus,  and  possibly 
in  other  twinning  species,  the  production  of  dizygotic 
twins  is  practically  a  specific  character  and  is  therefore 
inherited.  The  inherited  character  is  some  physiological 
peculiarity  of  the  ovarian  rhythm  resulting  in  the 
synchronous  ovulation  of  an  egg  from  each  ovary.  In 
close  relatives  of  this  species  the  ovaries  alternate  in 
functioning,  so  that  the  right  ovary  would  function  in 
one  pregnancy  and  the  left  in  the  next.  The  breaking 
up  of  this  alternating  rhythm  in  Euphractus  and  the 
establishment  of  a  synchronic  rhythm  in  the  two 
ovaries  would  appear  to  involve  a  profound  alteration 
of  the  general  metabolism.     That  this  altered  condition 


RELATION  TO  GENERAL  BIOLOGICAL  PROBLEMS    119 

is  established  as  an  inherited  specific  character  is  the 
only  conclusion  that  the  facts  admit. 

The  question  of  whether  twinning  in  ruminants  is 
hereditary  is  dealt  with  by  various  writers.  The  ex- 
periments of  Alexander  Graham  Bell  furnish  the  best 
available  evidence  on  this  point.  In  1889  he  purchased 
a  farm  at  Beinn  Bhreagh  in  Nova  Scotia  and  found 
himself  in  possession  of  a  flock  of  sheep.  In  the  spring 
of  1890  one-half  of  the  lambs  born  on  the  place  turned 
out  to  be  twins.  Evidently  he  had  a  race  of  sheep 
with  an  unusually  strong  twinning  tendency,  and  it 
became  an  object  of  interest  to  discover  just  which 
ewes  were  twin-bearing  and  whether  they  showed  any 
easily  recognizable  correlated  characters.     Bell  says: 

Upon  examining  the  milk-bags  of  the  sheep,  a  peculiarity  was 
observed  which  was  thought  might  be  significant.  Normally, 
sheep  have  only  two  nipples  upon  the  milk-bag,  but  in  the  case 
of  several  of  the  sheep  examined,  supernumerar>'  nipples  were 
discovered  which  were  embryonic  in  character  and  not  in  a 
functional  condition.  Some  had  three  nipples  in  all,  and  some 
four.  Of  the  normally  nippled  ewes  24  per  cent  had  twin  lambs; 
but  of  the  abnormally  nippled  43  per  cent  had  twins.  The  total 
number  of  ewes,  however,  was  so  small  (only  51)  as  to  deprive  the 
percentage  of  much  significance.  Still  the  figures  suggested  a 
possible  correlation  between  fertility  and  the  presence  of  super- 
numerary nipples,  and  it  seemed  worth  while  to  make  an  extended 
series  of  experiments  to  ascertain  (i)  whether,  by  selective  breeding, 
the  extra  nipples  could  be  developed  so  as  to  become  functional, 
and  (2)  whether  the  ewes  possessing  four  functional  nipples  instead 
of  two  would  turn  out  to  be  more  fertile  than  other  sheep  and 
have  a  larger  proportion  of  twins. 

Apparently  no  difficulty  was  experienced  in  develop- 
ing the  embryonic  supernumerary  nipples  into  func- 
tional organs,  and  by  1904  nearly  all  of  the  ewes  on  the 


I20  THE  BIOLOGY  OF  TWINS 

farm  possessed  four  functional  nipples  and  a  practically 
pure  line  of  sheep  with  four  milk-bearing  nipples  was 
established.  Subsequently  many  five-  and  six-nippled 
sheep  appeared  and  occasionally  seven-  and  eight-nippled 
ones  were  produced.  The  result  was  due,  not  to  the 
selection  of  fluctuating  variations,  but  to  the  appearance 
of  mutations  and  the  selection  of  these  suddenly  appear- 
ing new  types  as  the  parents  of  succeeding  generations. 
The  attempt  to  establish  a  correlation  between 
supernumerary  nipples  and  twin-bearing  was  dis- 
appointing.    In  a  subsequent  paper  Bell  says: 

At  first  it  appeared  that  four-nippled  ewes  were  less  fertile 
than  ordinary  sheep,  for  they  had  a  smaller  proportion  of  twins; 
but  this  turned  out  to  be  due  to  the  fact  that  the  process  of 
selection  had  necessarily  resulted  at  first  in  a  flock  composed 
mainly. of  young  ewes,  and  young  sheep  rarely  have  twins.  After 
the  four-nippled  ewes  had  grown  to  full  maturity  they  were  found 
to  be  as  fertile  in  this  respect  as  ordinary  sheep,  if  not  more  so. 

The  four-nippled  stock  proved  a  failure  in  so  far  as  twinning 
was  concerned,  so  in  1909  the  flock  was  cut  down  to  six-nippled 
ewes  alone.  There  were  indications  in  191 2  that  the  six-nippled 
stock  will  ultiniately  turn  out  to  be  twin-bearers,  as  a  rule,  when 
they  become  fully  mature. 

Whether  or  not  this  expectation  was  realized  I  am 
not  in  a  position  to  say.  On  the  whole,  it  does  not 
appear  that  the  production  of  a  twinning  race  of  sheep 
by  selecting  for  supernumerary  nipples  is  a  success,  for 
at  best  only  a  very  general  correlation  between  the  twin- 
bearing  tendency  and  the  tendency  to  extra  nipples 
exists. 

A  much  more  promising  method  of  producing  a 
twin-bearing  race  of  sheep  would  be  to  breed  exclusively 
from  twin  individuals  irrespective  of,  or  in  correlation 


RELATION  TO  GENERAL  BIOLOGICAL  PROBLEMS    121 

with,  the  extra-nipple  character.  Bell  had  in  mind 
such  a  procedure  when  he  wrote  his  191 2  communica- 
tion, but  has  not,  so  far  as  I  am  aware,  carried  it  out. 
He  noted,  however,  several  interesting  facts  that  might 
increase  the  hereditary  tendency  to  produce  twins. 
^'Twin-bearing  ewes  are  on  the  average  much  heavier 
than  single-bearing  ewes."  The  condition  of  the 
mother  at  time  of  mating  is  also  important,  for  when 
the  mother  is  fat  and  in  prime  physical  concHtion  the 
percentage  of  twins  is  larger  than  when  the  mother  is 
lean.  Ewes  mated  in  October  when  the  pasturage  is 
at  its  best  have  a  much  larger  proportion  of  twins  than 
those  mated  later  in  the  breeding  season  when  the 
pasturage  is  on  the  wane.  Very  few  ewes  mated  in 
December  have  twins.  A  lowered  nutrition  of  the 
mother  after  mating  in  October  favors  carrying  twins 
successfully  to  birth,  as  it  keeps  the  size  of  fetuses 
rather  small  and  thus  obviates  undue  crowding. 

We  may  conclude  from  Bell's  experiments  that 
twinning  is  distinctly  a  hereditary  character  in  sheep 
which  is  not  merely  sporadic  but  more  or  less  racial; 
a  fairly  large  percentage  of  twins  appears  to  be  specific 
for  sheep,  but  this  percentage  may  be  greatly  enhanced 
by  selective  breeding  from  twinning  strains.  The 
economic  importance  of  establishing  a  twinning  race 
of  sheep  is  obvious,  for,  as  Bell  says,  ''if  the  farmers 
could  raise  two  lambs  instead  of  one  for  e\er\'  ewe 
wintered,  sheep  breeding  in  Nova  Scotia  might  become 
a  profitable  industry  of  great  importance." 

Although  there  is  a  widespread  belief  among  breeders 
that  twinning  in  cattle  is  hereditary,  there  is,  so  far 
as  I  am  aware,  no  direct  evidence  that  such  is  the  case. 


122  THE  BIOLOGY  OF  TWINS 

No  experiments  of  the  sort  carried  out  upon  sheep  by 
Bell  have  been  made  upon  cattle.  Doubtless  similar 
results  could  be  obtained,  although  the  normal  percent- 
age of  twins  in  cattle  is  very  much  smaller  than  in 
sheep.  Even  if  the  ratio  of  twin  births  in  cattle  could 
be  increased  by  selective  breeding,  the  advantages  of 
such  an  increase  would  be  considerably  reduced  by 
the  frequency  of  sterile  freemartins. 

There  are,  however,  certain  rather  indirect  evidences 
that  dizygotic  twinning  is  hereditary  in  cattle.  Several 
cases  of  cows  producing  several  sets  of  twins  have  been 
recorded  by  various  writers.  One  interesting  case, 
cited  by  Pearl,  is  that  of  a  cow  that  produced  suc- 
cessively three  single  offspring,  then  two  pairs  of  twins, 
next  triplets,  then  a  single  calf,  and  finally  the  set  of 
triplets  shown  in  the  photograph  (Fig.  40).  The 
middle  calf  is  a  normal  male  and  the  two  outside  ones 
are  freemartins.  The  male  is  a  typical  Guernsey  like 
the  dam,  but  the  freemartins  are  therefore  like  their  sire. 
The  two  freemartins  are  remarkably  alike  and  are  be- 
lieved by  Pearl  to  be  monozygotic.^  Cases  of  triplets 
are  extremely  rare,  but  Cole  records  seven  cases  among 
303  plural  births.  Since  plural  births  in  cattle  occur 
in  only  about  2  per  cent  of  births,  triplets  occur  only 
about  once  in  2,000  cases.  Possibly  many  more  triplet 
gestations  begin,  but  result  in  the  death  or  early  abortion 
of  one  or  more  members  of  the  set. 

Since  no  experiments  have  ever  been  performed  by 
way  of  selecting  for  a  twinning  strain  of  human  beings, 
the  only  evidence  of  a  hereditary  tendency  in  twinning 

^  In  another  place  I  have  shown  the  improbabihty  of  monozygotic 
twinning  in  cattle. 


RELATION  TO  GENERAL  BIOLOGICAL  I'RGiiLLALS    123 

is  statisticaL  Danforth'  in  a  popular  article  conccrninj^ 
the  heredity  of  twinning  has  collected  data  that  may 
readily  be  interpreted  as  indicating  that  the  tendency 
is  not  merely  sporadic,  but  has  a  congenital  basis. 
Fifty  pairs  of  newborn  twins  were  found  to  have  171 
singly  born  and  10  twin  older  brothers  and  sisters, 
a  ratio  of  1:18.  In  mothers'  fraternities  (i.e.,  bro- 
thers and  sisters)  there  were  318  single  births  and  ten 
pairs  of  twins  (1:32),  and  in  fathers'  fraternities  219 
single  and  eight  pairs  of  twins  (1:37).  When  these 
ratios  are  compared  with  the  normal  incidence  of  twins 
(1:90.6),  it  appears  that  certain  strains  are  more 
hable  to  twins  than  others;  this  implies  a  hereditary 
tendency. 

I  am  well  acquainted  with  a  family  in  which  strik- 
ingly  similar  duplicate  twins  occurred  in  two  con- 
secutive births.  The  first  pair  were  females,  the  second 
pair  males.  A  collection  of  data  of  this  sort  would  be 
very  interesting,  as  it  would  tend  to  indicate  that 
monozygotic  twinning  is  hereditary-;  no  other  such 
data  are  available  to  me. 

Whether  the  tendency  to  twinning  is  a  factor  resident 
in  the  mother  or  the  father  is  not  clear.  That  the 
twinning  factor,  whatever  it  may  be,  might  reside  in 
the  father  is  suggested  by  an  extraordinary  case  cited 
by  R.  Berger.^  The  case  concerns  a  man  whose  tirst 
wife  had  quadruplets  once  and  twins  ten  times;  his 
second  wife  had  triplets  three  times  and  twins  ten 
times.  The  man  was  the  father  of  sixty-eight  children. 
Dr.  Berger  inferred  that  the  tendency  to  twinning  is 

'C.  H.  Baniorth,  Journal  of  Heredity,  VII  (1916)- 
^ Zentralblatt  f.  Gyndkologie,  X  (1914). 


124  THE  BIOLOGY  OF  TWINS 

due  rather  to  the  father  than  to  the  two  mothers. 
How  such  a  condition  could  be  determined  by  the  male 
is  difficult  to  imagine,  but  it  may  well  be  that  the 
tendency  to  polyembryonic  development  of  an  egg 
might  be  stimulated  by  some  peculiarity  of  the  sperm. 
If  the  twins  in  this  case  were  all  duplicates,  this  would 
be  an  interesting  possibihty;  but  the  father  could 
hardly  control  double  or  triple  ovulation  in  the  mother. 
Unfortunately  no  data  are  given  as  to  the  sex  of  the 
twins  in  this  striking  case. 

It  will  readily  be  seen  that  monozygotic  and  dizy- 
gotic twinning  involve  totally  different  situations  and 
would  therefore  doubtless  be  inherited  in  quite  difTer- 
ent  ways,  if  inherited  at  all.  Danforth  is  inclined  to 
believe  that  the  ability  to  produce  twins  is  common 
to  all  strains,  and  that  there  is  merely  a  variation  in 
frequency  of  incidence  of  twins  in  different  strains. 
The  only  method  of  settling  the  question  is  that  of 
collecting  a  really  adequate  mass  of  data;  no  such 
collection  is  yet  at  hand. 

Both  monozygotic  and  dizygotic  (including  poly- 
zygotic)  twinning  are  characters  capable  of  being 
inherited  as  unit  characters  and  of  being  made  racial 
or  specific  by  selective  breeding.  In  cattle  and  probably 
also  in  man  twinning  seems  to  be  recessive  and  single 
births  dominant;  hence  a  pure  twinning  strain  could 
be  produced,  if  at  all,  only  by  interbreeding  homozygous 
recessive  individuals,  that  is,  males  and  females  that 
are  the  offspring  for  at  least  two  generations  of  ancestors 
that  were  themselves  twins. 


CHAPTER  VII 
VARIATION  AND  HEREDITY  IN  TWINS 

A.      VARIATION   AND   HEREDITY   IN  ARMADILLO 

QUADRUPLETS 

The  known  laws  of  heredity  and  theories  as  to  the 
mechanism  of  inheritance  are  at  present  appHcable 
only  to  those  usual  reproductive  modes  which  involve 
the  development  of  but  a  single  offspring  from  a  fertilized 
egg.  A  new  situation  appears  when  an  egg  gives  rise 
to  two  or  more  offspring,  and  new  modes  of  inheritance 
are  involved. 

If  polyembryonic  offspring  were  absolutely  identical 
within  a  monozygotic  set,  the  problem  would  be  to 
account  for  the  identity.  Since  they  are  not  identical, 
but  show  a  definite  variability  within  a  set,  the  problem 
is  to  account  for  the  differences  that  exist  among  them. 

Only  in  one  character  are  the  members  of  a  poly- 
embryonic set  always  identical:  they  are  always  of  the 
same  sex.  In  all  other  respects  intra-set  differences  of  a 
more  or  less  radical  character  exist.  Some  of  these 
differences  are  purely  extrinsic  in  nature  or  origin; 
others  are  strictly  intrinsic  or  due  to  factors  inherited 
from  the  parents. 

The  following  problems  connected  with  heredity  in 
monozygotic  quadruplets  present  themselves  for  solu- 
tion: (a)  What  kinds  of  character  are  inherited  ?  (7)) 
Which  ones  are  inherited  by  all  and  which  are  subject 
to  unequal  distribution  among  the  fetuses  of  ditTerent 

125 


126  THE  BIOLOGY  OF  TWINS 

sets?  (c)  According  to  what  laws  are  such  characters 
inherited  ?  In  the  briefest  possible  way  an  attempt 
will  be  made  to  present  an  outline  of  some  of  the  studies 
on  heredity  in  armadillos  which  have  been  published  in 
several  recent  papers.' 

MATERIALS   FOR   THE    STUDY   OF   HEREDITY 

No  other  animal  is  so  beautifully  adapted  as  is  the 
armadillo  for  the  detailed  and  accurate  comparison  of 
parent  and  offspring  or  of  offspring  among  themselves. 
The  strikingly  diagrammatic  arrangement  of  the  integu- 
ment into  five  armor  shields  (see  frontispiece),  each 
shield  consisting  of  well-defined  units  (scutes  or  scales), 
furnishes  an  unparalleled  opportunity  for  the  study  of 
inter-  and  intra-individual  correlation.  These  charac- 
ters, moreover,  are  definitive  in  number  and  arrange- 
ment long  before  birth.  How  fortunate  a  circumstance 
that  this  species,  which  has  so  unique  a  method  of 
reproduction,  should  also  possess  equally  unique  pos- 
sibilities for  biometric  treatment! 

Although  any  part  of  the  armor  would  serve  well 
the  purposes  in  hand,  the  banded  region,  on  account 
of  its  regularity  and  clean-cut  character,  seems  almost 
made  to  order  for  biometric  study.  Each  band  is 
made  up  of  from  50  to  70  units,  here  called  scutes.  A 
scute  consists  of  a  horny  scale,  a  bony  base,  and  a 
definitely  arranged  group  of  hairs.  For  our  purposes 
this  whole  complex  may  be  viewed  as  a  unit  character. 
Studies  have  been  made  of  the  specific  variability  in 
total  numbers  of  scutes  in  the  nine  bands  and  of  that 

^  H.  H.  Newman,  Biological  Bulletin,  XXIX,  Nos.  i  and  2;  ibid.^ 
Journal  of  Experimental  Zoology,  XV,  No.  2. 


VARIATION  AND  HEREDITY  IN  TWINS         127 

for  each  band.  These  studies  are  a  necessary  pre- 
liminary for  determinations  of  the  coefficients  of  correla- 
tion among  quadruplet  sets,  but  need  not  be  dealt  with 
here.  To  illustrate  the  ways  in  which  heredity  works 
in  polyembryonic  species  only  the  most  essential  facts 
about  the  modes  of  inheritance  of  these  scute  groups 
need  be  presented. 

In  all  of  this  w^ork  a  limitation  upon  any  complete 
analysis  of  the  situation  is  imposed  by  the  fact  that  it 
has  been  possible  to  study  the  heredity  from  one  parent 
only.  Breeding  in  captivity  was  not  found  feasible 
for  several  reasons.  First,  so  large  a  number  of  sets 
was  required  for  statistical  study  that  it  w^ould  have 
been  an  enormous  undertaking  to  capture  and  keep  the 
necessary  number  of  parents.  Secondly,  attempts  to 
keep  armadillos  in  confinement  showed  that,  as  a  rule, 
they  become  badly  diseased  and  die.  Thirdly,  in  order 
to  obtain  knowledge  of  the  pairing  and  symmetry  rela- 
tions of  the  embryos,  it  was  found  necessary  to  remove 
the  unborn  fetuses  from  their  mothers;  this  involved 
killing  the  mothers,  a  practice  hardly  feasible  in  the  case 
of  painstakingly  domesticated  animals.  Finall}',  in  the 
few  cases  where  offspring  were  born  in  captivity,  it  was 
found  that  the  mothers  ate  the  offspring,  thus  totally 
nullifying  the  results  of  breeding  experiments.  Con- 
sequently our  method  of  capturing  and  killing  preg- 
nant females,  removing  and  preserving  the  fetuses,  and 
also  preserving  the  armature  of  the  mothers  for  compari- 
son with  those  of  the  fetuses,  gave  the  maximum  results 
possible  wath  the  material. 

This  limitation  of  the  study  of  heredity  to  the 
maternal  side  only  is  less  of  a  disadvantage  than  might 


128  THE  BIOLOGY  OF  TWINS 

at  first  appear,  for  the  armor  characters,  which  are  the 
most  available  features  for  study,  are  the  same  in  both 
sexes.  There  being  no  sex-dimorphism  on  the  basis 
of  these  characters,  the  inheritance  from  mothers  is 
equally  strong  for  male  and  for  female  offspring.  Con- 
sequently we  have  every  reason  to  believe  that  what 
we  discover  about  the  inheritance  from  the  maternal 
side  would  prove  to  be  the  same  as  that  from  the  paternal 
side  if  the  latter  were  known. 

Inheritance  of  the  nujnbers  of  scutes.^ — -The  first  study 
of  inheritance  of  scute  numbers  was  based  upon  a 
comparison  of  the  total  number  of  scutes  in  the  nine 
bands  in  mother  and  in  quadruplet  offspring.  A  few 
t}^e  cases  may  first  be  cited: 

Type  I.  Maternal  number  dominant:  Set  C  4.  Mother  has 
574  scutes;  fetus  I,  574;  fetus  II,  579;  fetus  III,  571;  fetus  IV, 
576. — Obviously  all  four  fetuses  are  extremely  close  to  the  mother 
in  this  character.  The  resemblance  is  exact  in  the  great  majority 
of  individual  bands. 

Type  II.  Paternal  number  dominant  in  all  four  fetuses:  Set 
K  73.  Mother,  541;  fetus  I,  521;  fetus  II,  518;  fetus  III, 
523;  fetus  IV,  520. — Obviously  none  of  the  fetuses  have  in- 
herited scute  numbers  from  the  mother.  Presumably  they  have 
inherited  it  from  the  father,  which  had  probably  about  520 
scutes. 

Type  III.  Maternal  number  present  in  some  of  the  fetuses 
but  not  in  others.     Three  subtypes  may  be  cited: 

I.  Set  K  54.  Mother,  565;  fetus  I,  565;  fetus  II,  576; 
fetus  III,  569;  fetus  IV,  568. — This  set  shows  three  like  the 
mother  and  one,  presumably,  more  like  the  father. 

^  A  study  of  the  variability  in  numbers  of  scutes  in  the  banded  region 
for  a  large  sample  of  the  species  shows  that  a  range  of  from  517  to  625 
scutes  and  an  average  deviation  from  the  mean  of  15  scutes.  Compare 
with  the  low  variability  within  the  monozygotic  sets. 


VARIATION  AND  HEREDITY  IX    IWIXS         129 

2.  Set  K  13.  Mother,  565;  fetus  I,  575;  fetus  11,  570; 
fetus  III,  563;  fetus  IV,  563.— Here  we  have  one  pair  like  the 
mother;    the  other  pair,  presumably,  like  the  father. 

3.  Set  C  29.  Mother,  561;  fetus  I,  549;  fetus  11,  560; 
fetus  III,  547;  fetus  IV,  546. — Here  we  have  one  like  the  mother 
and  three,  presumably,  like  the  father.  In  this  connection  it  is 
interesting  to  recall  the  relations  of  fetuses  shown  in  Figs.  2 1  and  22, 
where  it  appears  that  three  fetuses  may  be  derived  from  one  half 
of  the  ectodermic  vesicle  and  one  from  the  other;  we  would  expect 
the  three  from  one  side  to  be  much  alike  and  the  one  from  the 
other  side  to  be  somewhat  different. 

Type  IV.  None  of  the  fetuses  are  at  all  closely  like  the  mother 
and  still  show  wide  differences  among  themselves:  Set  C  65. 
Mother,  543;  fetus  I,  561;  fetus  II,  563;  fetus  III,  573;  fetus  IV, 
573. — The  paternal  number  is  probably  at  least  as  high  as  573; 
the  others  are  partly  paternal  and  partly  maternal.  Examination 
shows  that  the  first  5  bands  are  much  like  those  of  the  mother  in 
fetuses  I  and  II,  while  the  last  4  bands  are  much  like  those  of 
fetuses  III  and  IV,  which  are  probably  purely  paternal. 

It  is  not  rare  to  find  good  cases  of  types  I  and  II 
which  shovv^  nearly  pure  maternal  or  paternal  dominance 
of  all  nine  bands.  Nearly  25  per  cent  of  cases  could  be 
classed  in  each  group.  The  remaining  cases  fall  in  the 
classes  that  show  the  various  inter-  and  intra-individual 
distributions  of  maternal  and  paternal  dominance.  One 
comes  to  the  conclusion  that  the  armor  characters  are 
examples  of  mosaic  or  particulate  inheritance.  The 
maternal  influence  is  largely  dominant  in  some  sets, 
or  in  some  individuals  of  a  set,  and  tlie  paternal  in 
others.  There  are  also  cases  in  which  the  intluence  of 
one  parent  predominates  in  some  bands,  and  that  of  the 
other  parent  in  other  bands.  Finally,  there  are  some 
cases  in  which  one  lateral  half  of  the  body  has  ([uite  a 
different  number  of  scutes  from  the  other  half,  and  one 


I30  THE  BIOLOGY  OF  TWINS 

of  these  halves  resembles  the  maternal  condition.  For 
tables  showing  all  the  data  upon  which  this  discussion 
is  based  the  reader  is  referred  to  a  special  paper  on  the 
inheritance  of  numbers  of  scutes.^  There  a  study  of 
inheritance  was  made  for  each  of  the  five  armor  regions. 
What  has  been  said  for  the  banded  region  applies  equally 
well  to  the  other  four  shields. 

A  general  conclusion  from  this  intricate  and  exten- 
sive mass  of  statistical  data  is  that  both  large  and 
small  groups  of  integral  variates,  such  as  the  aggregate 
of  scutes  in  an  armor  shield  or  a  single  band,  are  inherited 
primarily  according  to  the  Mendelian  laws  of  dominance, 
with  only  a  minor  degree  of  blending,  and  that  the 
dominance  is  regional  and  not  very  often  general  for  a 
large  section  of  armor.  This,  then,  is  a  sort  of  particu- 
late inheritance,  a  mode  of  inheritance  which  implies  a 
somatic  segregation  of  parental  characters. 

When  biometrical  methods  of  inheritance  study 
are  applied  to  this  material,  certain  new  facts  are 
brought  out,  but  certain  other  facts  already  seen  for 
individual  cases  are  reduced  to  general  terms  and  are 
likely  to  be  lost  sight  of. 

Correlation  between  mothers  and  of  spring  as  to 
numbers  of  scutes. — -By  the  use  of  standard  statistical 
methods  the  coefficients  of  correlation  between  mothers 
and  offspring  have  been  derived  for  a  large  number  of 
characters.  To  illustrate:  it  was  found  that,  with 
respect  to  the  total  number  of  scutes  in  the  banded 
region  of  armor,  there  was  a  coefficient  of  correlation 
between  mothers  and  56  sets  of  male  quadruplets  of 
0.5522=1=0.0625,  and  for  59  sets  of  female  quadruplets, 

^  H.  H.  Newman,  loc.  cit. 


VARIATION  AND  HEREDITY  IN  TWINS         131 

0.5638=1=0.0597.  Making  allowance  for  i)ro]Kibility 
of  error,  the  coefficients  of  both  sexes  are  practically 
identical;  the  coefficient  is  about  0.5,  which  is  ]\\>i 
what  we  should  expect  if  mother  and  father  contributed 
equally  to  the  inheritance  of  these  characters.  Pre- 
sumably, then,  the  coefficient  for  fathers  and  offspring 
would  also  be  o.  5  or  thereabout.  It  will  be  noted  also 
that  the  males  inherit  from  mothers  just  as  strongly 
as  do  the  females,  which  goes  to  show  that  we  can  ignore 
sex  in  dealing  with  the  inheritance  of  scute  characters. 
Practically  the  same  degree  of  correlation  exists  for 
the  number  of  scutes  in  the  other  four  parts  of  the 
armor.  All  of  this  evidence  simply  means  that,  on  the 
average,  embryos  show  as  much  paternal  dominance 
in  scute  numbers  as  they  do  maternal.  Remembering, 
then,  that  the  degree  of  resemblance  between  mothers 
and  their  polyembryonic  sets  of  offspring  is  about  0.5, 
it  will  be  interesting  to  compare  w^ith  this  the  degree  of 
resemblance  among  the  quadruplets  themselves. 

Correlations  among  individuals  of  quadruplet  sets  as  to 
numbers  of  scutes.- — ^Using  the  same  statistical  methods  as 
in  determining  the  heredity  between  mother  and  offspring, 
we  find  that  for  total  numbers  of  scutes  in  the  banded 
region  there  is  a  coefficient  of  correlation  for  56  sets  of 
male  quadruplets  of  0.9294=1=0.0057,  and  for  59  sets 
of  female  quadruplets,  0.9129=1=0.0059.  This  degree 
of  correlation  is  extremely  high  and  it  has  no  paral- 
lel among  interindividual  correlation  coefficients.  In 
fact,  the  members  of  quadruplet  sets  are  as  strikingly 
alike  (as  closely  correlated)  as  are  the  paired  organs  of 
single  individuals  or  as  the  right  half  of  a  single 
individual  is  like  the  left.     The  correlation  constants 


132  THE  BIOLOGY  OF  TWINS 

brought  out  for  the  numbers  of  scutes  in  the  banded 
region  are  practically  duplicated  by  those  derived 
from  a  study  of  any  other  part  of  the  armor.  Results 
of  this  sort  would  in  themselves  be  sufficient  to  prove 
the  polyembryonic  character  of  armadillo  quadruplets; 
if  these  individuals  were  from  four  germ-cells  there 
would  be  no  reason  to  expect  correlations  higher  than 
those  that  obtain  between  brothers,  which  are  never 
much  higher  than  0.5.  From  the  genetic  standpoint 
a  set  of  armadillo  quadruplets  is  essentially  one  individ- 
ual in  four  parts.  When  we  are  comparing  one  fetus 
of  a  set  with  another,  we  are  dealing  with  a  special 
case  of  intra-individual  correlation.  The  proof  of  this 
assertion  lies  in  the  fact  that  the  degree  of  correlation 
is  of  the  same  order  as  those  determined  for  equivalent 
parts  of  one  individual  and  is  never  paralleled  by  that 
determined  between  separate  individuals. 

In  closing  this  very  brief  statement  of  some  of  the 
significant  facts  about  the  inheritance  of  aggregates  of 
integral  variates,  two  points  should  be  emphasized. 
First,  it  should  be  said  that  these  scute  number  dif- 
ferences are  about  the  only  inherited  differences  found 
in  the  species  that  are  available  for  biometric  study. 
Armadillos  are  practically  all  alike  in  color,  size  for 
age,  and  proportions  of  body  parts.  Males  and  females 
are  also  alike  except  for  the  genitalia.  In  view  of  these 
circumstances  the  reader  will  readily  appreciate  my 
choice  of  characters  for  the  study  of  heredity.  What- 
ever facts  about  heredity  are  to  be  discovered  for  this 
species  must,  therefore,  be  discovered  in  connection 
with  these  scutes  of  the  armor.  The  second  point  to 
remember  concerning  the  armor  characters  is  that  they 


VARIATION  AND  HEREDITY  IN  TWINS         133 

are  inherited  in  mosaic  fashion.  There  is  every  evidence 
of  an  unequal  distribution  of  inherited  units  among  the 
cleavage  products  of  the  single  germ-cell.  This  results 
in  an  interindividual  segregation  of  inherited  characters, 
which  is  much  like  the  unilateral  appearance  of  a  color 
character  in  certain  piebald  types,  where  one  half  of 
the  face  is  colored,  the  other  half  white,  or  where  one 
foreleg  is  colored  and  the  other  not.  This  somatic 
segregation  of  inherited  characters  is  very  general  and 
will  be  discussed  more  at  length  in  a  subsequent 
connection. 

Inheritance  of  double  bands. — The  arrangement  of  the 
scutes  of  the  banded  region  is  in  general  remarkably 
regular  (see  frontispiece).  Each  band  is  typically  com- 
posed of  a  single  row  of  scutes.  A  small  percentage  of 
individuals  show  an  irregularity  in  scute  arrangement 
consisting  of  parts  of  bands  that  are  double,  while  the 
rest  are  single.  In  Fig.  42  a  number  of  t>^es  of  band 
doubling  found  in  a  single  set  of  quadruplets  are  shown. 
Sometimes  the  double  part  is  quite  extensive,  involving 
a  large  part  of  the  band;  sometimes  it  consists  of  a 
doubling  of  only  one  scute  (third  band  from  top  to 
right,  Fig.  42).  All  intermediate  conditions  are  found. 
Band  doublings  may  be  confined  to  one  half -band,  or 
they  may  be  repeated  on  both  halves;  i.e.,  they  may 
be  bilateral  or  unilateral  in  their  expression.  The 
presence  of  irregularities  of  this  sort  furnishes  us  with 
the  only  data  by  means  of  which  we  can  get  a  really 
definite  idea  of  the  distribution  of  inherited  characters 
among  polyembryonic  offspring.  It  was  noted  quite 
early  in  comparing  the  individuals  of  polyembrsonic 
sets  that  sometimes  band  doublings  were  repeated  with 


134  THE  BIOLOGY  OF  TWINS 

striking  faithfulness  of  position  and  detail  in  two  or 
more  individuals  of  a  set  and  were  totally  absent  in 
others  of  the  same  set.  Sometimes  all  four  individuals 
showed  these  characters,  but  to  a  very  different  extent 
or  in  different  positions.  For  example,  the  doubling 
might  be  unilateral  in  one  pair  of  twins  and  bilateral 
in  the  other,  or  the  character  might  involve  a  dozen 
scutes  in  some  and  only  one  or  two  in  others.  The  real 
significance  of  this  situation  did  not  become  apparent 
until  it  was  found  that  these  somewhat  anomalous 
arrangements  of  scutes,  which  in  general  we  call  ''dou- 
blings," were  definitely  inherited.  It  appears  that  there 
is  a  close  genetic  relation  between  "scute  doublings," 
where  the  anomaly  affects  only  one  armor  unit  (an 
incipient  doubling),  and  "band  doubling,"  where  from 
two  to  many  units  are  involved.  Sometimes  band 
doubling  in  the  mother  is  inherited  in  the  offspring  as 
scute  doubling,  and  vice  versa.  Quite  often  the  expres- 
sion of  the  character  may  differ  within  the  set  of  off- 
spring, so  that  a  band  doubling  or  a  scute  doubling  in 
the  mother  may  be  inherited  in  some  offspring  as  a 
band  doubling  and  in  others  as  a  scute  doubling. 

In  general  it  may  be  stated  that  in  all  cases  except 
two,  which  are  quite  doubtful,  when  a  mother  has 
either  a  scute  or  a  band  doubling,  one  or  more  offspring 
in  a  set  show  a  doubling.  There  are  three  categories 
in  one  homogeneous  collection  of  140  sets  of  quad- 
ruplets, in  which  the  condition  of  the  mother  is  definitely 
known : 

(a)  Those  in  which  both  mother  and  offspring  show 
doubHng;  of  these  there  are  56  sets,  29  female  and 
27  male. 


VARIATION  AND  HEREDITY  IN  TWINS         135 

(h)  Those  in  which  the  mother  is  normal  (without 
doubhng),  but  in  which  doubhng  occurs  among  the 
offspring;  of  these  there  are  41  sets,  of  which  22  are 
female  and  19  male;  in  the  group  it  is  assumed  that 
the  father  possessed  the  factors  for  doubling. 

{c)  Those  in  which  both  mother  and  offspring  are 
entirely  without  doubling;  of  these  there  are  43  sets, 
of  which  22  are  female  and  21  male;  in  this  group  it 
Hcust  be  assumed  that  the  father  possessed  no  doubling 
factors. 

These  data  would  appear  to  show  conclusively  that 
the  ''doubling"  factor  is  inherited  as  a  dominant,  but 
that  the  mode  of  inheritance  is  not  typically  MendeHan ; 
if  '^ doubling"  is  dominant  and  ''lack  of  doubling"  is 
recessive,  we  should  expect  a  considerable  number  of 
individuals  to  be  heterozygous  for  "doubling"  and  to 
produce  equal  numbers  of  germ-cells  that  carry  the  dou- 
bling factor  and  of  those  that  do  not.  If  heterozygosity 
occurred,  we  should  often  get  "non-doubling"  in  sets  of 
offspring  from  mothers  that  show  doubhng  but  are  hetero- 
zygous for  the  character.  That  this  result  is  not  realized 
is  a  strange  circumstance,  and  one  that  can  be  explained 
satisfactorily  only  on  the  assumption  that  a  segregation 
of  dominant  and  recessive  factors  occurs  during  cleavage 
so  that  the  blastomeres,  which  go  to  produce  both  soma 
and  germ-cells  of  the  individuals,  are  pure  for  the  factor 
in  question,  and  that  homozygous  offspring  are  always 
produced,  and  never  any  heterozygous  ones.  Segrega- 
tion like  that  which  is  supposed  to  occur  during  matura- 
tion divisions,  when  chromosome  reduction  accompanies 
it,  would  appear  to  occur  here  during  the  process  of 
cleavage  in  which  no  reduction  of  chromosomes  takes 


136  THE  JBIOLOGY  OF  TWINS 

place.  But  before  entering  upon  a  discussion  of  somatic 
segregation,  we  must  needs  study  some  of  the  data  upon 
which  the  theory  is  based.  Only  a  few  selected  cases  may 
be  presented  within  the  scope  of  the  present  volume ;  the 
reader  is  referred  for  complete  data  to  two  recent  papers.^ 
■  For  convenience  of  presentation  it  is  necessary  to 
conventionalize  the  figures  of  band  and  scute  doubling. 
The  methods  of  representing  band  doublings  together 
with  the  detail  of  actual  cases  are  shown  in  Fig.  42. 
At  the  top  of  the  page  is  shown  a  single  band  with  an 
extensive  doubling  involving  all  but  a  few  marginal 
units  at  the  right  and  the  left.  Directly  underneath 
is  a  conventional  representation  with  numbers  indicating 
the  numbers  of  scutes  involved.  Below  this  is  another 
type  of  doubling  in  detail,  with  the  conventional  repre- 
sentation just  beneath.  Various  types  of  scute  doubling 
are  shown  also,  and  the  method  of  indicating  the  location 
and  distribution  of  them  is  by  placing  a  small  diagram 
of  a  double  scute  in  a  band  and  locating  its  position 
with  reference  to  margin  or  middle  by  a  number.  In 
doubtful  cases  an  arrow  points  to  the  spot  from  which 
the  count  proceeds. 

The  character  and  distribution  of  inherited  band 
and  scute  doubling  may  be  illustrated  by  a  complete 
detail  drawing  of  the  affected  bands  in  one  set  of  fetuses 
(set  K  87)  and  their  mother  (Fig.  42).  The  affected 
band  of  the  mother  is  marked  M  and  those  of  the  four 
fetuses  I,  II,  III,  and  IV.  Fetuses  I  and  II  (a  pair) 
have  each  two  affected  bands.  It  will  be  seen  that  the 
mother  has  a  unilateral  doubling  involving  13-14 
scutes  six  places  from  the  left-hand  margin,  of  band  i. 

^  H.  H.  Newman,  loc.  cit. 


60 


60 


4 


G 

iZ 

29 

8 

6" 

IZ 

8 

8  7/                      i 

3 

4 

16' 

9-68                          1 

4- 

/Z 

6' 

Fig.  41. — Various  kinds  of  band  doublings,  and  conventional  methods 
of  representing  them.  The  detail  of  a  double  band  is  shown  in  /  and 
a  conventionalized  diagram  of  the  same  band  in  2.  Another  kind  of  band 
doubling  is  shown  in  detail  in.  j,  and  the  same  in  diagram  in  4.  Simi- 
larly, 5  is  a  detail  of  scutes  and  6  a  detail  of  un(lcrl\ing  bony  plates. 
In  7  are  diagrams  of  two  adjacent  bands  with  similar  doubling  in  both; 
8:71  and  9:68  mean  band  S  with  a  total  of  71  scales  and  band  9  with  OS 
scales.  The  other  numbers  indicate  the  number  of  scutes  in  the  various 
regions. 


138 


THE  BIOLOGY  OF  TWINS 


All  four  fetuses  have  a  more  or  less  extensive  doubling 
of  the  same  band.  In  fetuses  I,  II,  and  IV  the  doubling 
is  bilateral,  but  the  more  extensive  doubling  is  on  the 


Fig.  42. — A  detail  drawing  of  all  the  bands  that  show  doubling  in 
mother  and  offspring  of  set  K  87.  if  =  mother;  I,  II,  III,  IV,  the  four 
fetuses. 


VARIATION  AND  HEREDITY  IN  TWINS         139 

left  side;  in  fetus  III  it  is  unilateral  but  on  the  side 
opposite  to  that  of  the  mother.  Fetuses  I  and  II  have 
in  addition,  in  bands  4  and  5,  respectively,  a  double 
scute  each.  In  fetus  I  the  double  scute  is  near  the 
right  margin;  in  fetus  II  it  is  near  the  left  margin. 
This  type  of  symmetry  reversal  is  very  common  between 
individuals  of  a  pair  and  is  called  mirror-imaging. 
Fetus  I  (belonging  to  one  pair)  has  the  more  extensive 
doubling  on  the  left  side,  and  the  opposite  fetus,  III, 
(belonging  to  another  pair)  has  all  the  doubling  on  the 
right  side;  this  is  an  example  of  mirror-imaging  involving 
embryos  derived  from  opposite  sides  of  the  egg.  That 
this  is  a  genuine  case  of  inheritance  of  doubling  can 
scarcely  be  doubted  when  it  is  considered  that  band 
doubling  occurs  in  only  about  3  per  cent  of  individuals 
in  the  species.  Its  occurrence  in  mother  and  four  off- 
spring is  more  than  a  coincidence. 

Set  K  30  (female  fetuses)  further  illustrates  the 
mode  of  inheritance  of  scute  and  band  doubling.  A 
somewhat  simplified  method  of  representing  the 
anomalies  is  here  adopted  (Fig.  43). 

In  the  mother  there  is  in  band  i  a  minimal  band 
doubling  involving  two  scutes  located  six  places  from 
the  left  margin.  In  band  2  there  is  a  scute  doubling 
ten  places  to  the  left  of  the  middle.  Some  kind  of 
doubling  appears  in  band  i  of  all  four  fetuses.  Fetus  I 
has  a  double  scute  in  exactly  the  same  spot  where  the 
mother  has  an  incipient  double  band;  fetus  II  has 
in  the  exact  middle  of  the  band  a  short  band  doubling 
of  three  scutes  and  a  double  scute  14  places  to  the  left 
of  the  middle,  or  nearly  in  the  place  where  the  mother 
has  a  double  scute  in  band  2.     In  addition,  fetus  II 


140 


THE  BIOLOGY  OF  TWINS 


has  a  double  scute  in  band  5  quite  close  to  the  middle 
or  almost  directly  in  a  line  with  the  band  doubling 


Fig.  43. — A  detail  drawing  of  band  doubling  in  set  K  30 

farther  forward.     Fetus  III  has  a  double  scute  exactly 
in  the  middle  of  band  i ;   and  its  partner,  fetus  IV,  has 


VARIATION  AND  HEREDITY  IN  TWINS         141 

a  band  doubling  involving  seven  scutes  exactly  in  the 
middle  of  band  i.  Nothing  could  show  more  clearly 
the  genetic  equivalence  of  scute  and  band  cKjubling, 
for  obviously  the  three  doublings  exactly  in  the  middle 
of  band  i  in  three  fetuses  (II,  III,  IV),  one  involving  one, 
another  three,  and  another  seven  scutes,  are  genetically 
equivalent  and  must  be  inherited  from  the  condition 
in  the  mother.  It  is  quite  common  to  find  positional 
reversals  involving  a  shift  from  near  the  margin  to  the 
middle.  This  type  of  symmetry  reversal  is  scarcely 
mirror-imaging,  but  is  nevertheless  of  a  kindred  charac- 
ter, doubtless  due  to  similar  factors.  It  will  be  noted 
that  fetus  I  has  the  same  position  of  the  doubling 
as  the  mother  has  in  the  incipient  band  doubling,  while 
fetus  II  has  inherited  the  double  scute  of  band  2  of  the 
mother  in  its  band  i.  Fetuses  III  and  IV  (a  natural 
pair)  have  a  positional  reversal  of  that  of  the  mother, 
III  inheriting  a  scute  doubling  in  the  same  place  where 
fetus  IV  inherits  a  band  doubling.  Again,  it  will  be 
seen  that  only  the  primary  fetuses  II  and  IV  inherit 
the  band  doubling,  while  the  two  secondary  individuals 
inherit  the  scute  doubling.  Fetus  II  is  the  only  one 
that  inherits  both  scute  and  band  doubling  and  has  the 
doubling  involving  two  bands,  as  in  the  mother. 

Set  K  4  (male  fetuses)  shows  another  set  of  con- 
ditions (Fig.  44).  The  mother  has  in  band  i,  beginning 
seven  places  from  the  left  margin,  a  small  band  (lou])ling 
of  three  scutes.  Fetus  I  has  in  band  i,  four  places  to  the 
left  of  the  middle,  a  somewhat  more  extensive  band 
doubhng  involving  five  scutes.  This  is  evidently  a 
case  of  inheritance  with  symmetry  reversal  of  one  side. 
All  the  other  fetuses  have  extensive  band  doubling.     In 


142 


THE  BIOLOGY  OF  TWINS 


fetus  III  the  doubling  involves  the  whole  band,  for 
there  is  an  extra  or  tenth  band  present;  in  fetus  I  the 
whole  band  is  double  except  four  scutes  on  the  left 


n 


I 


I 


Fig.  44. — A  detail  drawing  of  band  and  scute  doubling  in  set  K  4 


margin;  and  in  fetus  IV  the  whole  is  double  except  four 
scutes  on  the  left  and  five  on  the  right.  Fetuses  I 
and  II  (a  pair)  are  alike  in  being  unilateral  in  doubling, 


VARIATION  AND  HEREDITY  IN  TWINS  143 

and  III  and  IV  (a  pair)  are  both  bilateral.  Perhaps 
the  most  striking  feature  illustrated  by  this  set  is  the 
great  variation  in  the  extent  of  doubhng  that  may 
appear  in  the  four  fetuses  of  a  single  set  where  there 
can  be  no  question  as  to  the  common  genetic  basis 
of  the  more  and  of  the  less  extensive  expression  of  the 
character. 


// 


Fig.  45. — Diagram  of  band  doubling  in  set  A  64 

By  way  of  contrast,  however,  we  shall  present  a  case 
in  which  the  mother  showed  no  doubling,  but  in  which 
the  fetuses  show  a  remarkable  degree  of  resemblance 
in  the  position  and  extent  of  the  anomaly.  Presumably 
the  condition  has  been  inherited  from  the  father.  Fig.  45 
is  a  diagrammatic  representation  of  the  doubling  of  bands 
in  Set  A  64.  Fetus  I  has  the  entire  first  band  double, 
for  there  are  ten  bands.  Fetus  II  has  a  beautifully  sym- 
metric bilateral  doubling  of  band  i,  consisting  of  fifteen 
double  scutes  situated  three  places  from  the  right  and 


144 


THE  BIOLOGY  OF  TWINS 


from  the  left  margin.  It  will  be  noted  that,  while  both 
fetuses  of  this  pair  are  bilaterally  symmetrical  in  the 
anomaly,  fetus  I  has  complete  doubling  and  fetus  II  has 
incomplete  doubling.  Fetuses  III  and  IV  have  absolutely 
identical  bilateral  doubling,  but  are  asymmetrical  in  that 
the  left  side  of  each  has  the  type  of  doubling  in  every 
detail  of  II,  while  the  right  side  of  each  is  completely 
double  like  the  condition  in  fetus  I. 

These  cases  of  band  doubling  must  serve  as  samples 
in  that  they  show  practically  all  the  peculiarities  involved 


Fig.  46. — Diagrammatic  representation  of  doublings  in  set  A  loi 

in  band  doubling  except  those  in  which  only  one,  two, 
or  three  of  the  fetuses  in  the  set  show  a  doubling.  One 
case  of  that  sort  must  be  cited. 

Set  A  1 01  (male  fetuses)  is  a  case  in  which  one  pair 
has  band  doubling  and  the  other  has  no  doubling  at  all 
(Fig.  46).  Fetus  I  has  an  exact  bilaterally  symmetrical 
anomaly  of  band  i,  involving  three  double  regions,  two 
lateral  ones  of  nine  scutes  each,  six  scutes  from  the 
margins,  and  a  median  doubling  of  seventeen  scutes. 
Fetus  II  has  the  band  double  except  six  scutes  on 
the  left  margin.     We  must  consequently  inquire  why 


VARIATION  AND  HEREDITY  IN  TWINS         145 

doubling,  evidently  inherited  from  the  father,  since 
the  mother  showed  none,  could  be  confined  to  the  two 
fetuses  and  excluded  from  the  other  two,  when  all  came 
from  the  same  fertilized  egg. 

Data  on  the  inheritance  of  scute  doubling. — Scute 
doubhng  is  far  more  frequent  in  its  incidence  than  band 
doubling,  but  the  modes  of  inheritance  and  distribution 
are  the  same.  Band  doubling  may  be  inherited  as 
scute  doubling,  or  vice  versa.  A  few  examples  of  the 
inheritance  of  scute  doubhng  will  be  sufficient  to  illus- 
trate the  principles  involved.  Impressive  cases  of 
resemblance  between  mother  and  offspring  and  among 
the  quadruplets  of  a  set  are  of  frequent  occurrence. 

Set  K  27  (Fig.  47)  is  one  of  the  most  remarkable 
cases  of  exact  inheritance.  The  mother  has  two  double 
scutes  located  in  definite  places  in  two  different  bands. 
One  of  the  oft'spring  (fetus  I)  has  two  identical  double 
scutes  in  exactly  the  same  places  in  the  same  two 
bands  as  in  the  mother.  The  other  three  fetuses  are 
without  any  doubling. 

Set  C  26  (Fig.  48).  The  mother  has  a  double  scute 
of  a  peculiar  kind  two  places  from  the  left  margin  of 
band  2.  The  paired  fetuses  (III  and  IV)  each  have 
identical  double  scutes  in  exactly  the  same  position  in 
band  i. 

Many  other  cases  occur  of  close  resemblance  between 
mother  and  offspring  involving,  however,  a  reversed 
symmetry.     An  example  will  illustrate  this: 

Set  C  76  (Fig.  49).  The  mother  has  in  band  6  a 
double  scute  five  places  from  the  left  margin.  Fetus 
III  has  a  similar  double  scute  in  band  4  five  places 
from  the  right  margin. 


146 


THE  BIOLOGY  OF  TWINS 


Set  C  30  (Fig.  50).  The  mother  has  in  band  i  a 
double  scute  two  places  from  the  right  margin,  and 
fetuses  II,  III,  and  IV  have  each  a  double  scute  two 
places  from  the  left  margin  of  the  same  band. 

M 


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Fig.  47 


z-ez 


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Fig.  48 


Q.1&  ^ 


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Fig.  49 

Figs.  47,  48,  and  49. — Showing  the  incidence  of  scute  doubling  in 
sets  K  27,  C  26,  and  C  76,  respectively.     (Details  in  text.) 

Mirror-imaging  among  the  fetuses  of  a  set. — It  is 
very  common  to  find  symmetry  reversals  within  a  set 
of  quadruplets  involving  what  we  call  mirror-imaging. 

Set  C  30  (Fig.  50).  Fetus  I  has  a  pecuhar  type  of 
scute  three  places  from  the  right  margin  of  band   i, 


VARIATION  AND  HEREDITY  IN  TWINS         147 


while  its  partner  has  an  identical  double  scute  two 
places  from  the  left  of  band  i.  Fetuses  III  and  VI 
(the   other  pair)    have   the   same   element   distributed 


C  30C^ 


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Fig.  si 

Figs.  50,  51. — Showing  the  incidence  of  scute  doubling  in  sets  C  30 
and  K  70.     (Details  in  text.) 

bilaterally  but  in  band  i  on  the  left  and  in  band  2  on 
the  right. 

Set  K  70  (Fig.  51).  Fetus  I  has  a  double  scute 
three  places  from  the  left  margin  of  band  6,  while  fetus 
III,  belonging  to  the  opposite  pair,  has  a  similar  element 


148  THE  BIOLOGY  OF  TWINS 

3  places  to  the  right  of  band  7.  Note  that  these  individ- 
uals are  not  members  of  a  pair,  but  are  situated  back  to 
back  on  opposite  sides  of  the  vesicle. 

Half-hand  reversals. — When  in  one  fetus  a  double 
scute  occurs  on,  say,  the  right  side  near  the  margin  and 
in  another  fetus  of  the  same  set  a  similar  element  occurs 
on  the  same  half -band,  but  near  the  middle,  there  is  said 
to  be  a  half -band  reversal.  Such  conditions  are  exactly 
like  the  occasional  reversals  of  pattern  of  the  index  fingers 
of  duplicate  human  twins  (see  Fig.  54),  where  the  right 
hands  of  the  two  show  a  reversal  or  mirror-image  sym- 
metry in  one  finger. 

Set  C  76  (see  Fig.  49).  Fetus  IV  has  a  double  scute 
twelve  places  to  the  left  of  the  middle  of  band  4.  Fetus 
II  has  a  similar  element  in  band  i  twelve  places  from 
the  right  margin.  Note  that  if  these  two  fetuses  were 
placed  back  to  back  (their  normal  position  in  the  vesicle) 
the  left  halves  would  be  mirror-image  duplicates  in  so 
far  as  the  double  scute  is  concerned. 

Many  more  examples  of  extremely  close  resemblance 
among  the  fetuses  of  a  set  might  be  cited,  but  for 
further  particulars  the  reader  is  referred  to  the  complete 
accounts  published  elsewhere.  Suffice  it  to  say  that 
the  number  of  cases  in  which  the  double  scute  is  unequiv- 
ocally inherited  is  so  large  that,  even  when  the  location 
of  the  element  in  mother  and  offspring  and  among  the 
fetuses  of  a  set  is  quite  diverse,  we  must  still  conclude 
that  the  character  is  inherited,  but  lacks  the  localization 
factor. 

This  leads  to  a  brief  discussion  of  the  possible  factorial 
basis  of  scute-  and  band-doubling  inheritance.  There 
appear  to  be  at  least  four  unit  factors  involved:    (i)  a 


VARIATION  AND  HEREDITY  IN  TWINS         J 4c) 

general  factor  for  doubling;  (2)  several  minor  factors 
determining  in  double  scutes  the  particular  kind  of 
doubling;  (3)  a  factor  determining  extent  of  dcjubling, 
whether  one  element  or  many  are  involved  in  doubling; 
(4)  a  localizing  factor,  determining  more  or  less  precisely 
the  exact  position  of  the  doubling  in  the  individual. 
It  may  be  supposed  by  way  of  illustration  that  a  mother 
has  the  doubhng  factor,  together  with  the  extension 
factor;  the  father  lacks  the  doubling  and  extension 
factor,  but  has  the  localizing  factor.  The  factors  could 
be  variously  distributed  among  the  four  fetuses  so  as  to 
give  all  sorts  of  combinations.  It  seems  quite  evident, 
then,  that  doubling  is  a  complex  matter,  not  at  all  a 
simple  unit  factor,  and  it  must  therefore  be  explained 
on  the  basis  of  the  interaction  of  several  factors. 

Somatic  and  germinal  segregation. — That  band  and 
scute  doubling  are  definitely  heritable  is  proved  by  the 
fact  that  doubling  is  always  present  in  some  form  in  the 
offspring  of  mothers  that  show  doubling.  The  real 
problem  is  to  find  a  mechanism  to  explain  why  it  so 
often  happens  that  some  of  the  fetuses  derived  from 
a  single  egg  exhibit  the  character  and  others  do  not. 
In  monozygotic  quadruplets  we  should  expect  the  same 
genetic  constitution  in  each  individual  unless  there 
exists  some  segregative  mechanism,  resulting  during 
early  ontogeny  in  an  irregular  distribution  of  the  factors 
responsible  for  doubhng.  It  has  been  suggested  that 
the  real  differentiating  factors  are  environmental; 
but  the  onlv  environmental  differences  conceivable  for 
monochorial  quadruplets  are  nutritional  differences 
due  to  more  or  less  extensive  placentation.  It  can 
be    shown,    however,    that    nutritional    differences,    so 


I50  THE  BIOLOGY  OF  TWINS 

great  as  to  have  a  pronounced  effect  on  size  and  stage 
of  development  of  different  individuals  of  a  set,  have  no 
effect  on  the  inheritance  of  doubling.  There  are 
several  quadruplets  in  which  the  various  individuals  are 
pronouncedly  different  in  size,  but  are  practically 
identical  in  the  character  and  incidence  of  doubling. 
The  differentiating  factor,  therefore,  must  be  within 
the  embryo  itself.  It  seems  logical  to  look  to  the 
cleavage  mechanism  as  the  probable  seat  of  the  irregu- 
lar distribution  to  different  areas  of  the  blastoderm  of 
the  factors  of  doubling.  Presumably,  with  an  ideally 
accurate  cleavage  mechanism  there  would  result  an 
exactly  identical  incidence  of  doubling  in  all  four  fetuses 
of  a  given  set.  That  identity  in  doubling  is  not  realized 
argues  strongly  for  unequal  distribution  {or  somatic 
segregation)  of  factors  during  cleavage.  Moreover,  since 
doubling  is  evidently  as  strongly  inherited  from  father 
as  from  mother,  it  seems  probable  that  nuclear  elements 
are  chiefly  involved,  for  the  cytoplasm  of  the  sperm  cell 
is  so  small  in  amount  as  to  be  negligible. 

If  we  assume  that  the  beginning  at  least  of  segrega- 
tion occurs  in  the  first  and  second  cleavages,  we  may 
suppose  that  when  the  factor  goes  to  the  first  two 
blastomeres  it  is  pretty  certain  to  appear  in  both  pairs 
of  fetuses;  when  it  goes  to  each  blastomere  of  the  four- 
cell  stage  it  is  likely  to  appear  in  all  four  fetuses.  If, 
however,  the  distribution  of  the  factor  is  such  that  it 
goes  entirely  to  one  of  the  first  two  blastomeres  and  not 
to  the  other,  we  should  expect  doubling  to  appear  in 
only  half  of  the  fetuses;  if,  again,  only  one  of  the  four 
cells  gets  the  factor,  we  should  have  the  factor  in  only 
one  quadrant  of  the  blastocyst  and  hence  should  prob- 


VARIATION  AND  HEREDITY  IN  TWINS         151 

ably  find  doubling  in  only  one  fetus.  This  final  resort 
to  the  early  cleavages  as  the  probable  mechanism 
responsible  for  the  distribution  of  doubling  factors  in 
polyembryonic  sets  is  taken  advisedly  after  a  thorough 
canvass  of  all  other  possibihties  which  have  suggested 
themselves.  The  idea  is  not  incompatible  with  the 
observations  that  have  led  to  the  budding  h}potheses 
of  Patterson,  as  has  already  been  shown,  but  fits  in 
better  with  the  fission  theory  of  polyembryony. 

Segregation  of  unit  factors  during  cleavage  may  be 
termed  somatic  segregation  whether  or  not  poly- 
embryony be  involved.  Somatic  segregation  must,  I 
believe,  take  place  in  those  forms  that  exhibit  mosaic  or 
particulate  inheritance;  in  a  spotted  animal,  showing  in 
some  areas  the  paternal  and  in  others  the  maternal  color, 
we  certainly  have  a  simple  case  of  somatic  segregation. 

It  is  in  connection  with  bud  variations  or  clonal  varia- 
tions, however,  that  we  find  a  condition  more  nearly  anal- 
ogous to  what  appears  to  happen  in  armadillo  embryos. 
A  green  plant  with  colored  flowers  may  produce  as  a 
bud  variation  a  white-leaved  branch  with  white  flowers. 
If  such  a  white  flower  were  self-fertilizing  and  produced 
good  seed  from  which  white  plants  could  be  reared,  we 
should  have  more  than  mere  somatic  segregation,  since 
germinal  segregation  has  gone  hand  in  hand  with  somatic. 
I  have  been  informed  that  just  such  a  situation  is 
sometimes  realized  for  plants. 

Now  in  the  armadillo  the  four  fetuses  are  produced 
agamically  as  fission  products^,  they  therefore  constitute 
a  clone.  There  is  every  opportunity  for  clonal  varia- 
tion, and  when  a  clonal  individual  gets  a  factor  that 
makes  for  doubling  in  the  soma,  it  always  has  gametes 


152  THE  BIOLOGY  OF  TWINS 

pure  for  this  character.  Otherwise  why  does  a  mother 
with  doubhng  always  have  offspring  that  show  some 
form  of  doubhng  ? 

Parallel  somatic  and  germinal  segregation  can  be 
explained  only  on  the  assumption  that  the  segregation 
mechanism  operates  at  a  period  prior  to  the  differentia- 
tion of  germinal  and  somatic  cells,  and  this  must  be 
during  the  early  cleavage  stages. 

Mirror-imaging  and  symmetry  reversals  as  the  result 
of  polyemhryony. — So  frequent  is  the  occurrence  of 
mirror-imaging  among  armadillo  quadruplets  that  it 
must  be  conceived  as  causally  related  with  polyembry- 
onic  development.  The  two  phenomena  are  so  closely 
related  that  it  is  my  belief  that  the  occurrence  of  sym- 
metry reversal  or  mirror-imaging  in  twins  or  double 
monsters  may  safely  be  taken  as  a  criterion  of  their 
monozygotic  origin.  Only,  however,  when  there  appears 
some  asymmetric  feature  like  unilateral  doubling  is 
mirror-imaging  recognizable.  If  both  sides  of  twin 
individuals  are  exact  bilateral  duplicates  no  symmetry 
reversals  would  be  possible;  hence  we  must  depend  upon 
the  unilateral  appearance  of  such  features  as  doubling 
for  signs  of  reversed  symmetry  relations. 

All  grades  of  mirror-imaging  are  found  in  armadillo 
quadruplets.  There  may  be  mirror-imaging  between 
individuals  of  opposite  pairs,  but  this  is  much  less 
common  than  imaging  between  twin  partners  derived 
from  one  half  of  the  Qgg. 

A  still  more  frequent  type  of  mirror-imaging  is  seen 
between  the  antimeric  halves  of  a  single  individual. 
These  facts  lead  to  the  conclusion  that  the  symmetry 
relations  among  quadruplets  are  the  result  of  an  intri- 


VARIATION  AND  HEREDITY  IN  TWINS         153 

cate  interplay  of  three  grades  of  successively  operating 
symmetry  systems,  the  later  tending  to  obliterate  the 
effect  of  the  earlier,  but  not  always  successfully.  In 
general,  mirror-imaging  between  opposites  is  evidence 
of  a  residuum  of  a  primary  bilateral  symmetry  that 
held  sway  in  the  blastocyst  before  polyembryonic 
fission  began.  When  the  primary  outgrowths  are 
formed,  they  are  the  product  of  the  antimeric  halves 
of  the  first  embryo  and  should  therefore  show  mirror- 
image  relations.  But  a  partial  physiological  isolation 
of  the  two  halves  permits  a  certain  reorganization  or 
regulation  of  new  symmetry  relations,  which  tends  more 
or  less  completely  to  destroy  the  original  symmetry, 
yet  often  leaving  a  trace  of  the  latter.  Similarly,  when 
the  secondary  outgrowths  arise  between  the  primary 
ones  a  certain  residuum  of  the  primary  symmetry  may 
be  carried  over  that  frequently  manifests  itself  in 
mirror-imaging  between  twins  derived  from  one  half 
of  the  original  embryo.  Finally,  when  each  secondary 
outgrowth  organizes  its  own  bilateral  symmetry,  it 
tends  to  lose,  partially  at  least,  the  earlier  symmetry 
relations,  and  to  estabhsh  its  own  mirror-imaging  of 
right  and  left  sides.  In  some  cases  traces  of  all 
three  symmetry  systems  appear  in  a  single  set  of 
fetuses,  but  it  is  common  to  find  only  two  systems 
interacting. 

Mirror -imaging  between  individuals  limited  to  integu- 
mentary structures. — When  examples  of  s>Tiimetry  re- 
versal were  first  discovered  to  occur  so  frequently  in 
the  armor  characters  of  the  armadillo,  it  seemed  probable 
that  similar  reversals  would  be  found  in  the  visceral 
arrangements.     One  would  expect  to  find  occasionally 


154  THE  BIOLOGY  OF  TWINS 

the  heart  apex  turned  to  the  right  instead  of  to  the  left, 
and  the  greater  curve  of  the  stomach  turned  to  the 
right.  An  extensive  examination,  however,  shows 
that  no  visceral  reversals  occur.  In  his  book,  Problems 
of  Genetics^  Bateson  calls  attention  to  the  same  situation 
in  human  duplicate  twins  and  says:  ''If  anyone  could 
show  how  it  is  that  neither  of  a  pair  of  twins  has  trans- 
position of  the  viscera  the  whole  mystery  of  division 
would,  I  expect,  be  greatly  illuminated." 

From  what  we  know  of  the  process  of  polyembryonic 
fission  in  the  armadillo,  it  would  appear  that  the  most 
likely  solution  of  this  problem  lies  in  the  fact  that 
twinning  is  initiated  and  carried  out  in  the  ectoderm, 
and  the  endoderm  becomes  involved  only  passively  and 
considerably  later.  What  more  natural,  then,  than 
to  look  for  evidences  of  twinning — mirror-image  rever- 
sals— only  in  ectodermal  and  closely  associated  structures 
of  the  integument?  In  human  duplicate  twins  it  is 
true  also  that  reversals  are  confined  to  the  friction 
ridges,  which  are  quite  homologous  in  origin  with  the 
armor  characters  of  the  armadillo. 

If  Bateson  should  chance  to  read  this  suggestion 
as  to  the  reason  why  transposition  of  viscera  does  not 
occur  in  twins,  I  doubt  whether  he  would  admit  that 
''the  whole  mystery  of  division  is  hereby  greatly  illu- 
minated." It  is  an  interesting  fact,  however,  that  the 
ectoderm,  where  the  rate  of  metabolism  is  highest, 
should  take  the  lead  and  carry  out  the  fission  process, 
thus  imposing  the  results  of  fission  on  the  tissues  of 
lower  rate  of  metabolism,  the  endoderm  and  the  meso- 
derm. The  ectoderm  is  dominant  in  early  development 
and    produces    the    central    nervous    system    through 


VARIATION  AND  HEREDITY  IN  TWINS         155 

which  it  exercises  progressively  greater  dominance  as 
development  proceeds. 

B.    VARIATION   AND   HEREDITY   IN   HUMAN   TWINS 

No  studies  of  value  have  appeared  dealing  with 
variation  and  heredity  in  dizygotic  or  fraternal  twins; 
consequently  our  attention  must  be  directed  solely 
to  these  phenomena  in  duphcate  (presumably  mono- 
zygotic) twins.  Perhaps  the  strongest  evidence  in 
favor  of  the  idea  that  certain  human  twins  are  mono- 
zygotic comes  from  a  study  of  the  variability  or  lack 
of  variability  between  these  twins.  So  pronounced  is 
the  lack  of  variability  in  some  cases  that  such  twins  are 
called  ''identical."  A  treatment  of  human  twins 
paralleling  that  of  armadillo  quadruplets  will  show 
many  points  in  common. 

THE  DEGREE   OF  IDENTITY  IN  HUMAN  DUPLICATE   TWINS 

The  first  serious  attempt  to  determine  the  closeness 
of  resemblance  between  twins  was  made  by  Galton.  As 
early  as  1875  he  showed  his  appreciation  of  the  value 
of  such  a  determination;  this  comes  out  clearly  in  his 
paper  entitled  ''The  History  of  Twins,  as  a  Criterion 
of  the  Relative  Power  of  Nature  and  Nurture."  Gal- 
ton conceived  the  idea  that  the  degree  of  resemblance 
between  duplicate  twins  is  an  index  of  the  strength 
of  heredity  as  opposed  to  environment.  In  aUcm])ting 
a  minute  comparison  between  twins  he  found  them  so 
much  alike  that  he  had  to  resort  to  such  details  as  the 
patterns  the  friction  ridges  in  the  palms  and  soles. 

It  remained  for  Wilder,  however,  actually  to  carry 
out   this   study  which   Galton   formulated,   and   some 


156 


THE  BIOLOGY  OF  TWINS 


discussion  of  his  (Wilder's)  conclusions  appears  in  the 
last  few  pages  of  this  chapter. 

Wilder^ s  studies  of  friction- skin  patterns  in  the  palms 
and  soles  of  twins. — It  is  a  familiar  fact  that  the  surest 
method  of  identifying  human  beings  is  the  finger-print 
method  now  in  use  in  all  modern  detective  bureaus 
and  prisons.  The  method  is  based  on  the  wide  range 
of  diversity  in  the  details  of  the  friction-ridge  patterns 


'^)> 


Fig.  52. — Photograph  (from  Wilder)  of  the  left  sole-prints  of  a  pair 
of  duplicate  twins.  The  heavy  lines  are  lines  of  interpretation.  Note 
the  striking  similarity  amounting  almost  to  identity. 

of  the  palmar  surfaces  of  the  hands  and  feet.  No  two 
individuals  are  exactly  alike  in  all  details,  but  the 
resemblances  between  certain  types  of  twins  are  really 
surprisingly  close.  The  prints  of  the  left  soles  of  a 
pair  of  twins  studied  by  Wilder  show  identity  of  general 
pattern  (Fig.  52)  but  lack  of  identity  in  detail,  for  the 
exact  number  of  friction  ridges  in  corresponding  parts 
of  the  pattern  differs  in  the  two  individuals. 


VARIATION  AND  HEREDITY  IN  TWINS         157 

Wilder  has  devised  an  elaborate  method  of  classifying 
the  various  types  of  pattern  in  both  pahn  and  sole, 
and  an  equally  elaborate  system  of  condensed  formulae 
for  denoting  the  pattern  complex  of  any  individual. 
For  a  description  of  this  method  of  formulating  friction- 
skin  diversity  the  reader  is  referred  to  Wilder's  various 
papers  on  these  subjects.  It  would  lead  us  too  far 
afield  to  attempt  to  introduce  any  adequate  key  to  the 
study  of  human  palms  and  soles  in  this  place.  Suffice 
it  to  say  that  in  his  studies  of  fifteen  pairs  of  same-sexed 
twins  and  one  set  of  opposite-sexed  triplets  he  noted 
that  eleven  of  them  were  "identical"  in  palm  and  sole 
patterns  and  five  were  unlike.  The  conclusion  is  reached 
that  those  which  are  "identical"  are  monozygotic  and 
those  which  fall  considerably  short  of  identity  are  dizy- 
gotic. Since  there  is  no  knowledge  of  the  intra-uterine 
conditions  in  any  case,  this  reasoning  backward  from 
identity  to  origin  is  in  this  case  unjustified.  That  this 
kind  of  reasoning  may  lead  to  grave  error  is  shown  in 
connection  with  those  armadillo  quadruplets  in  which 
there  are  quite  pronounced  differences  between  the 
members  of  a  set;  yet  their  origin  from  a  single  egg-cell 
cannot  be  questioned.  In  certain  monozygotic  human 
twins  it  might  readily  happen  that  by  somatic  segrega- 
tion the  paternal  condition  of  friction  ridges  would  go 
to  one  individual  and  the  maternal  to  the  other;  or 
there  might  be  a  partial  segregation  of  important 
elements  of  the  pattern  so  that  they  would  be  dis- 
tributed differently  in  the  two. 

It  must  not  be  lost  sight  of,  however,  that  identity 
in  friction-ridge  patterns  of  twins  makes  their  mono- 
zygotic   origin    very   highly    probable.     Identity    may 


158 


THE  BIOLOGY  OF  TWINS 


demonstrate  monozygotic  origin,  but  lack  of  identity 
does  not  disprove  the  possibility  of  monozygotic  origin. 
Wilder  has  illustrated  certain  very  good  cases  of  identity 
by  means  of  photographs  of  palm-  and  sole-prints  (see 
Fig.  52).     The  heavy  lines  are  the  lines  of  interpretation 


Fig.  53. — Photograph  (from  Wilder)  of  the  left  (above)  and  right 
(below)  palm-prints  of  a  set  of  triplets.  Note  the  close  identity  of  the 
males,  which  are  evidently  "identicals,"  and  the  unlikeness  of  these 
to  the  female  triplet  on  the  right,  which  has  evidently  come  from  a 
separate  egg. 


and  serve  merely  to  emphasize  a  real,  fundamental 
identity.  Another  very  interesting  case  is  that  of  triplets 
(two  boys  and  one  girl,  Fig.  53)  and  illustrates  very  well 
the  difference  between  ordinary  ''fraternal"  resemblance 
and  true  identity.  The  palm-prints  of  the  girl  are  no 
more  like  those  of  the  two  boys  than  is  usually  the  case 


VARIATION  AND  HEREDITY  IN  TWINS         159 

for  ordinary  brothers  and  sisters,  but  those  of  the  two 
boys  are  extraordinarily  similar.  It  may  be  decided  that 
the  two  boys  are  monozygotic  and  that  the  girl  was 
derived  from  a  separate  zygote. 

The  most  significant  features  of  the  finger  prints 
of  duplicate  twins  are: 

1.  In  one  pair  of  twins  there  was  an  almost  complete 
mirror-imaging  of  the  two  palm  patterns  of  the  two 
individuals.  The  left  hand  of  x  corresponds  to  the  right 
h-and  of  y,  and  vice  versa.  This  case  corresponds  to  the 
rather  rare  mirror-imaging  of  "opposites"  seen  in 
armadillo  quadruplets,  and  goes  far  to  prove  that 
polyembryony  actually  occurs  in  man. 

2.  In  three  other  sets  symmetry  reversal  occurs 
to  a  limited  extent,  but,  curiously  enough,  always  in 
connection  with  the  pattern  of  the  index  fingers.  In 
two  cases  the  finger-prints  of  the  left  index'  finger  of  x 
and  of  y  show  a  reversed  s^Tnmetry  so  that  one  of  them 
mirrors  the  condition  in  the  left  index  finger  of  the 
other  twin.  In  the  third  case  the  symmetry  reversal 
is  in  the  right  index  finger.  These  cases  are  quite 
homologous  to  the  half-band  reversals  noted  for  arma- 
dillo quadruplets.  In  human  twdns  these  reversals  are 
fairly  numerous  considering  the  small  number  of  twins 
examined.  Fig.  54  shows  the  finger-prints  of  one  pair 
of  duplicate  twins  with  the  reversed  patterns  in  the  two 
upper  prints. 

"These  reversals  of  index  patterns,"  says  Wilder, 
"seem  to  occur  with  too  great  frequency  to  be  disposed 

'  An  interesting  parallel  exists  between  the  conditions  seen  in  man 
and  in  the  armadillo.  In  man  mirror-imaging  is  usually  confmcd  to  the 
index  finger,  while  in  the  armadillo  it  is  largely  confined  to  the  first 
band  of  armor. 


i6o 


THE  BIOLOGY  OF  TWINS 


of  as  a  lack  of  correspondence  with  the  rest."  Yet 
no  well-defined  idea  of  the  significance  of  these  reversals 
seems  to  have  occurred  to  Wilder.  Bateson,  however, 
sees  in  these  peculiar  phenomena  evidence  that  twins 

derived  from  a  single 
zygote  have  been  parts 
of  a  single  system  of 
symmetry.  This  is 
evidently  the  key  to 
the  significance  of 
symmetry  reversals,  as 
was  brought  out  in  the 
discussion  of  symmetry 
reversals  of  armadillo 
quadruplets. 

As  Wilder  has 
pointed  out  so  clearly 
in  his  latest  paper 
("Palm     and     Sole 

Studies  "0: 

The  bands  of  the 
armadillo  carapace,  with 
their  variation  and  the 
friction  ridges  of  human 
palms  and  soles,  are  par- 
tially  or   wholly   homolo- 


FiG.  54. — Prints  (from  Wilder)  of 
the  tips  of  the  first  three  fingers  of 
the  left  hand  of  a  pair  of  "identical" 
twins.  Note  the  reversed  symmetry 
of  the  index-finger  prints.  This  is 
a  good  case  of  mirror-imaging,  so 
characteristic  of  monozygotic  twins. 


gous  structures,  so  that 
their  use  in  determining  the  degree  of  similarity  of  twinned  indi- 
viduals is  equally  warrantable  in  both  cases,  while  the  results 
may  well  be  compared.  Both  deal  with  epidermic  structures, 
the  probable  homologues  of  reptilian  scales,  placed  in  rows;  in 
both  are  observed  the  similar  phenomena  of  the  forking  and  con- 


^  Biological  Bulletin,  XXX,  Nos.  2  and  3  (1916). 


VARIATION  AND  HEREDITY  IN  TWINS         i6i 

sequent  doubling  of  the  lines  thus  formed.  Even  the  double 
scale  in  the  armadillo  may  have  its  counterpart  in  a  twin  sweat- 
pore,  which  indicates  the  composite  nature  of  the  unit  to  which 
they  belong. 

Wilder  believes,  however,  that  the  far  greater 
complexity  of  pattern  in  the  human  friction  ridges 
gives  a  better  basis  for  detailed  comparison  than  the 
simpler  condition  seen  in  the  armadillo.  He  also 
admits  that  there  is  a  great  advantage  on  the  side  of 
the  armadillo  in  the  possibility  of  studying  the  embryonic 
conditions. 

It  is  remarkable  that  the  same  situations  of  exact 
resemblance  and  of  various  grades  of  symmetry  reversal 
occur  in  both  human  and  armadillo  monozygotic  twins. 
The  significance  of  these  phenomena  must,  I  believe, 
be  the  same  in  both  cases.  In  the  armadillo  these 
manifestations  are  the  result  of  polyembryonic  develop- 
ment;  it  is  almost  certainly  the  same  for  man. 

Coefficients  of  correlation  between  twins. — The  human 
friction  ridges  do  not  furnish  as  good  material  as  do  the 
bands  of  the  armadillo  for  establishing  an  exact  numer- 
ical measure  of  the  degree  of  resemblance  between  twins. 
An  enumeration  of  the  individual  friction  ridges  of  the 
numbers  of  sweat-pores  in  a  given  pattern  would  give 
results  comparable  in  availabihty  to  the  enumeration 
of  scales  in  the  banded  region  or  in  individual  bands  of  the 
armadillo.  Doubtless  these  data  could  be  obtained  for 
human  twins,  but  as  yet  there  is  no  such  information 
at  hand. 

Attempts  have  been  made,  however,  to  work  out 
the  percentage  differences  between  twins  on  the  basis 
of  a  comparison  of  various  physical  measurements  and 


1 62  THE  BIOLOGY  OF  TWINS 

weights.  Vernon,  for  example,  gives  the  data  for  two 
pairs  of  identical  twins,  one  of  which,  aged  twenty- 
three  years,  showed  an  average  percentage  difference 
for  a  number  of  characters  of  0.28;  while  the  other, 
aged  twelve,  showed  a  percentage  difference  of  0.71. 
Weismann  presented  the  data  on  one  pair  of  twin 
brothers,  aged  seventeen  years,  which  showed  a  per- 
centage difference  of  2.2,  nearly  ten  times  that  of 
Vernon's  first  pair. 

Wilder  obtained  numerous  measurements  of  three 
pairs  of  twins,  aged  respectively  21.10,  17.10,  and 
17.  II  years.  They  showed  an  average  percentage 
of  difference  of.  2.03.  Although  Wilder  realizes  that 
dimensional  and  other  physical  measurements  are 
quite  unsafe  criteria  of  the  genetic  resemblance  between 
twins,  he  is  unable  to  furnish  any  substitute  less  objec- 
tionable. It  is  well  known  that  nutritional  and  other 
environmental  differences  greatly  affect  the  size  and 
weight  of  individuals.  In  spite  of  these  facts,  however, 
the  percentage  differences  brought  out  for  twins  are 
quite  comparable,  in  so  far  as  they  show  close  resem- 
blance between  monozygotic  individuals,  with  the 
coefficient  of  correlation  brought  out  for  the  number  of 
scutes  in  the  bands  of  the  armadillo. 

It  should  be  pointed  out,  however,  that  fairly 
marked  dimensional  differences,  even  at  birth,  could 
not  be  used  as  evidence  against  the  monozygotic  origin 
of  any  particular  pair  of  twins;  in  the  armadillo,  where 
the  monozygotic  origin  is  specific  and  unequivocal, 
there  is  frequently  a  striking  size  difference  among  the 
quadruplets  of  a  given  set.  On  the  whole,  then,  it 
would  seem  inadvisable  to  use  dimensional  measure- 


VARIATION  AND  HEREDITY  IN  TWINS         163 

ments  either  at  birth  or  in  later  Hfe  as  data  for  determin- 
ing the  degree  of  resemblance  (coefficient  of  correlation) 
between  twins. 

Modes  of  inheritance  of  friction-ridge  patterns. — 
In  his  recent  studies  on  the  palms  and  soles  of  human 
beings,  Wilder  has  brought  out  some  very  interesting 
and  significant  facts  about  the  inheritance  of  certain 
of  these  patterns.  Without  going  into  the  details  of 
classification  or  codification  of  patterns,  it  may  be  said 
that  palm  and  sole  configurations  are  markedly  herit- 
able. A  comparison  of  the  prints  of  the  right  palm 
of  a  certain  father  and  his  six-year-old  son"^  show  how 
exactly  these  details  may  be  duplicated  in  two  successive 
generations.  Certain  elements  in  the  palm  pattern, 
for  instance,  seem  to  be  inherited  after  the  manner  of 
Mendelian  unit  characters,  strongly  marked  patterns 
of  unusual  types  being  dominant  over  less  marked  ones. 

An  interesting  possible  development  from  these 
facts  has  to  do  with  the  use  of  these  patterns  in  deter- 
mining the  parentage  where  there  is  doubt  about  the 
matter.  If  a  certain  unusual  type  of  pattern  were 
found  in  either  supposed  parent  and  the  same  pattern 
appeared  in  the  supposed  offspring,  the  relationship 
might  be  said  to  be  established  with  a  high  degree  of 
probabiHty.  If,  on  the  other  hand,  such  a  pattern 
failed  to  appear  or  appeared  only  in  a  highly  modified 
form,  the  conclusion  of  lack  of  relationship  would  not 
be  safe;  there  is  always  the  chance  that  a  combination 
pattern,  derived  by  a  mixing  of  the  patterns  of  the  two 
parents,  might  occur,  or  even  that  a  grandparental 
pattern,  recessive  for  one  generation,  might  reappear. 

^  See  Wilder,  loc.  cit. 


1 64  THE  BIOLOGY  OF  TWINS 

Much  more  study  is  needed  before  this  type  of  evidence 
could  be  used  as  a  rehable  method  of  estabhshing  rela- 
tionships in  man. 

Nevertheless,  it  is  significant  that  peculiar  patterns 
in  palm-  and  sole-prints  are  heritable  just  as  are  peculiar 
patterns  in  the  bands  of  armor  in  the  armadillo.  The 
various  modifications^  of  the  inherited  pattern  are  also 
like  the  various  expressions  of  doubling  in  armadillo 
scutes  and  bands.  The  patterns  may  be  reproduced  in 
a  more  pronounced  or  in  a  reduced  condition,  but  they 
appear  to  be  inherited  in  the  expected  proportions  on 
the  basis  of  their  unit  character  nature.  Unfortunately 
scarcely  any  information  has  been  secured  as  to  the 
direct  inheritance  of  palm  and  sole  patterns.  This 
field  would  be  well  worth  investigation. 

In  concluding  the  discussion  of  the  friction-ridge 
correspondences  it  will  be  of  interest  to  describe  a 
case  worked  out  for  a  pair  of  "conjoined"  twins  (pygo- 
pagi)  by  Wilder.  These  twin  girls  (Margaret  and 
Mary)  were  first  observed  a  day  or  two  after  birth  and 
are  now  over  four  years  old.  ''They  are  united  in  the 
sacro-iliac  region,  but  are  placed  somewhat  obliquely, 
so  that  instead  of  looking  in  opposite  directions  they 
are  rotated  about  45  degrees  toward  the  same  side." 
A  study  of  their  palms  and  soles  was  deemed  of  great 
interest,  since  there  appeared  to  be  no  question  as  to  their 
monozygotic  derivation.  The  palms  of  all  four  hands 
are  practically  alike  in  pattern;  the  right  hand  of  each 
one  not  only  mirrors  its  own  left  hand  but  also  the 
left  hand  of  its  twin  partner.  Since  there  is  no  asym- 
metry in  the  hands,  there  is  no  chance  to  observe 
symmetry  reversal  or  mirror-imaging.     Strange  to  say, 


VARIATION  AND  HEREDITY  IN  TWINS        i6s 


however,  although  both  sole  patterns  of  Mary  and  the 
left  one  of  Margaret  are  alike  (Fig.  55),  the  right  sole- 
print  of  Margaret  had  a  totally  different  configuration. 


Fig.  55. — Sole-prints  of  the  conjoined  twins  Margaret  (above)  and 
Mary  (below).  Note  that  the  right  of  Margaret  is  a  mirror-image  of 
the  left  of  Mary,  but  that  the  right  of  Mary  shows  a  totally  dillerent 
pattern  from  her  left.     (From  Wilder.) 

The  left  sole  of  Mary  and  the  right  sole  of  Margaret  are 
more  nearly  identical  than  are  the  right  and  left  soles 
of  Margaret,  which  is  a  good  case  of  mirror-imaging 


1 66  THE  BIOLOGY  OF  TWINS 

and  just  the  kind  of  thing  one  might  look  for  in  con- 
joined twins.  Quite  unexpected,  however,  is  the  occur- 
rence of  the  odd  pattern  in  the  right  sole  of  Mary. 

I  am  inclined  to  interpret  the  cause  of  this  aberrant 
sole  pattern  in  the  light  of  similar  conditions  found  in 
armadillo  quadruplets.  There  it  was  not  unusual  to 
find  that  one  or  more  fetuses  in  a  set  inherited  a  pecu- 
liarity while  others  did  not.  This  was  explained  as  an 
instance  of  somatic  segregation  taking  place  during  the 
early  cleavage  divisions.  Evidently  this  is  an  instance 
of  a  similar  phenomenon  occurring  in  conjoined  twins. 
In  principle  this  is  not  different  from  the  unilateral 
reappearance  in  an  ordinary  offspring  of  a  peculiarity 
inherited  from  a  parent.  The  case  is  one  of  very 
special  interest,  since  it  is  the  only  one  of  twins  on 
record  where  the  embryonic  membranes  were  studied  in 
correlation  with  the  somatic  resemblances.  It  was 
ascertained  that  "there  was  a  single  chorion  without 
trace  of  a  separating  partition,  and  the  placenta  was 
bilobed,  and  nearly  as  large  as  two  normal  placentae." 
The  umbilical  cord  was  single  for  ii  cm.  from  the  pla- 
centa, forking  into  two  branches,  one  a  little  larger 
than  the  other,  running  to  the  two  individuals.  It  is 
only  to  be  regretted  that  the  palm  and  sole  patterns 
of  the  two  parents  were  not  recorded.  Possibly  these 
data,  quite  crucial  in  character,  I  believe,  may  be  yet 
available;  it  would  probably  demonstrate  the  fact  of 
somatic  segregation  of  parental  characters. 

Variations  on  brain  convolutions  and  in  hair  arrange- 
ment in  duplicate  twins. — A  significant  paper  by  Sano^ 

^  F.  Sano,  Philosophical  Transactions  of  the  Royal  Society  of  London, 
CCVIII  (1916). 


VARIATION  AND  HEREDITY  IX  TWINS        167 

has  just  appeared  in  which  a  detailed  comparison  of  the 
convolutional  pattern  of  the  brains  of  a  pair  of  stillborn, 
full-term,  Belgian  war  babies.  These  were  adjudged, 
and  probably  correctly,  to  be  ''identical''  or  mono- 
zygotic twins,  although  there  appear  to  be  marked  dif- 
ferences in  size,  weight,  facial  and  cranial  indices,  size 
of  brain  and  of  head.  ''The  boy  called  A  has  a  more 
receding  forehead;  the  nose  is  more  turned  up;  the 
distance  between  the  root  of  the  nose  and  the  superior 
border  of  the  upper  lip  smaller  (6.5  mm.  v.  8.  mm.), 
hence  the  mouth  remains  open.  The  chin  is  more 
receding.  The  ear  of  A  is  closer  to  the  head,  has  very 
little  enrolment  of  the  border,  and  its  lobule  is  adherent, 
while  the  second  boy's  ear  is  more  unfolded  and  graceful." 

Although  in  these  and  other  respects  the  twins  are 
quite  strikingly  different,  they  are  alike  in  eyes,  in  lines 
and  furrows  of  the  hands,  and  in  other  inherited  char- 
acters. The  author  concludes  ''that  the  male  twins 
under  examination  are  very  similar  to  each  other  and 
also  to  their  mother.  No  essential  differences  were  to 
be  found." 

The  description  of  the  brains  of  the  twins  is  very 
detailed  and  technical,  but  we  may  accept  the  author's 
conclusion  that,  although  the  brain  of  twin  B  is  larger 
and  somewhat  more  advanced  in  structure  than  that 
of  twin  A,  they  are  strikingly  similar.  My  own  o])inion 
as  to  the  other  differences  pointed  out  by  Sano  is  that 
they  too  are  to  be  interpreted  as  the  result  of  a  difference 
in  the  degree  of  maturity  of  the  two  twins,  B  being 
distinctly  in  advance  of  A.  It  will  be  recalled  that 
armadillo  quadruplets  in  advanced  pregnancy  show 
equally  striking  differences  in  developmental  age. 


1 68  THE  BIOLOGY  OF  TWINS 

The  point  that  interested  me  most  in  Sano's  paper 
has  to  do  with  the  crown  whirls  of  the  two  twin  boys. 
That  of  A  is  to  the  left  of  the  median  line  and  that  of 
B  to  the  right.  In  detail  the  hair  whorls  of  the  two 
are  mirror-image  duplicates.  Such  a  condition  is  just 
what  we  might  expect  in  monozygotic  twins,  for  there 
is  an  intimate  relation  between  hair  arrangements, 
scale  arrangements,  and  friction-skin  patterns;  they 
are  all  integumentary  structures  and  are  therefore 
likely  to  exhibit  mirror-imaging.  Were  there  no  other 
evidence  of  the  monozygotic  character  of  these  twins, 
this  condition  of  the  hair  whorls  would  go  far  to  prove  it.^ 

That  many  of  the  differences  between  such  mono- 
zygotic twins  may  be  merely  the  result  of  differences 
in  developmental  age  is  shown  by  an  interesting  pair  of 
very  early  human  twins  that  were  studied  by  Dr.  F.  E. 
Chidester.  One  of  these  twins  was  about  as  advanced 
as  a  month-old  embryo  and  the  other  was  in  an  early 
primitive-streak  stage.  They  both  lay  in  a  single 
large  amnion  and  were  separated  by  a  small  area  ol 
extra-embryonic  tissue.  Dr.  Chidester  kindly  sho'/,ed 
me  a  drawing  of  a  surface  preparation  of  these  twins, 
but  only  a  detailed  study  of  sections  will  reveal  tiie 
interrelations  of  the  two,  and  I  shall  be  much  interested 
to  learn  the  outcome  of  this  study. 

Mental  resemblances  between  duplicate  human  twins. — 
If  twins  are  strikingly  alike  structurally,  it  follows  that 
they  must  be  alike  functionally,  since  similar  structures 
could    hardly    have    dissimilar    functions.     A    pair    of 

^I  have  just  learned  of  an  authentic  case  of  reversal  in  a  pair  of 
duplicate  twins,  one  of  whom  is  a  colleague  of  mine  here  in  the  Uni- 
versity of  Chicago.  One  of  these  twn  brethren  is  right-handed  and 
the  other  left-handed. 


VARIATION  AND  HEREDITY  IN  TWINS        169 

twins  with  identical  brains  should  have  identical  mental 
equipment  at  birth,  but  further  mental  development 
would  depend  on  training  and  environment.  Many 
accounts  have  been  given  of  the  remarkable  unity  of 
thought  and  action  of  duphcate  twins.  They  have 
been  described  as  speaking  in  unison  with  the  same 
inflection,  as  having  the  same  dreams,  etc.  Even 
more  remarkable  are  stories  to  the  effect  that  if  one 
twin  becomes  ill  the  other  does  also,  and  that  an  injury 
to  one  twin  is  felt  by  the  other.  Such  anecdotes, 
though  common,  are  probably  without  foundation. 
More  credible  are  stories  that  one  twin  got  two  baths 
and  the  other  none,  or  that  one  twin  was  spanked 
for  the  other's  misdeeds. 

THE   COMPARATIVE   POTENCY   OF   HEREDITY 
AND   ENVIRONMENT 

To  what  extent  and  within  what  limits  are  the 
definitive  characters  of  the  individual  determined 
at  the  time  of  fertilization,  and  in  how  far  are  the 
minutiae  of  organic  structure  to  be  considered  as  the 
product  of  individual  variabihty  beyond  the  limits  of 
hereditary  control  ?  This  very  fundamental  question 
has  been  raised  under  various  guises  for  many  years. 
It  is  the  old  problem  as  to  the  relative  potency  of 
heredity  and  environment  in  development,  or  that  of 
predetermination  versus  epigenesis. 

As  long  ago  as  1870  Galton  previsioned  the  impor- 
tance of  the  study  of  twins  as  material  probably  adapted 
to  a  solution  of  the  problem.  His  views  are  very 
clearly  stated  in  his  paper  ''The  History  of  Twins,  as 
a    Criterion    of    the    Relative    Power    of   Nature   and 


lyo  THE  BIOLOGY  OF  TWINS 

Nurture."  In  a  subsequent  paper  (1892)  he  shows  his 
continued  interest  in  this  problem  by  his  remark: 
*'It  may  be  mentioned  that  I  have  an  enquiry  in  view 
which  has  not  yet  been  fairly  begun,  namely:  to  deter- 
mine the  minutest  biological  unit  that  may  be 
hereditarily  transmissible.  The  minutiae  in  the  finger- 
prints of  twins  seem  suitable  objects  for  this  purpose." 
Wilder  in  his  paper  on  '' Duplicate  Twins  and 
Double  Monsters"  follows  up  this  clue  and  presents 
many  important  facts  as  to  the  close  resemblance 
between  twins  in  the  patterns  of  the  friction  ridges 
in  palms  and  soles.  The  conclusion  reached  is  as 
follows : 

The  influence  of  the  germ-plasm  and  its  mechanism  [i.e., 
the  direct  control  exercised  by  heredity]  is  exerted  upon  the 
friction-skin  surfaces  only  so  far  as  concerns  the  general  con- 
figuration, i.e.,  the  main  lines,  the  patterns,  and  other  similar 
features;  the  individual  ridges  and  their  details  [minutiae]  are 
apparently  under  the  control  of  individual  mechanical  laws  to 
which  they  are  subjected  during  growth.  Have  we  then  arrived 
at  the  limit  of  the  control  of  the  predetermining  mechanism 
beyond  which  mechanical  laws  are  alone  operative,  and  is  it 
then  possible  to  hold  that  the  modifications  in  the  latter  field  are 
the  results  of  individual  experience,  and  that  they  are  similar 
in  the  various  members  of  a  given  species  solely  because  of  similar 
environment?  To  these  and  similar  questions  we  can  give  no 
answer  at  present;  yet  it  seems  likely  that  in  general  in  the  palm 
and  sole  markings,  not  only  in  man  but  in  other  mammals  as 
well,  we  have  a  set  of  easily  observed  and  very  significant  data 
which  may  yield  important  results  to  future  investigators. 

Data  similar  to  that  on  the  friction-ridge  patterns 
of  human  twins  are  afforded  by  a  study  of  scute  and 
band  doubling  in  armadillo  quadruplets.  Just  as 
there   is   in   human    twins   usually   a   striking   general 


VARIATION  AND  HEREDITY  IN  TWINS        171 

similarity  between  the  main  patterns  of  the  two  individ- 
uals and  a  difference  in  the  exact  detailed  exi)ression  of 
the  pattern  in  terms  of  integumentary  units,  so  in 
armadillo  quadruplets  there  is  generally  a  pattern 
(double  band  or  double  scute)  in  a  similar  region  of 
the  armor  in  all  four  individuals  of  a  set,  but  there 
may  be  a  considerable  difference  in  the  number  and 
distribution  of  the  integumentary  units  employed  in 
the  expression  of  the  pattern.  The  pattern  (doubling) 
may  be  expressed  in  a  large  number  of  units  (scutes) 
in  some  members  of  a  quadruplet  set  and  in  a  small 
number  (sometimes  only  one)  of  units  in  others.  It 
may  be  expressed  unilaterally  in  some  and  bilaterally 
in  others.  Or,  finally,  the  pattern  may  be  expressed  in 
some  members  and  totally  suppressed  in  others. 

When,  in  his  examination  of  same-sexed  twins, 
Wilder  encountered  cases  in  which  certain  patterns 
were  not  sufficiently  identical  to  meet  his  preconceived 
ideas  of  what  duplicate  twins  should  be,  he  concluded 
that  they  were  fraternal  twins  (dizygotic).  In  the 
light  of  what  I  have  found  in  armadillo  quadruplets, 
which  are  unquestionably  monozygotic,  it  does  not 
seem  safe  to  exclude  from  the  category  of  monozygotic 
twins  those  that  fail  to  show  identity  of  pattern;  the 
same  practice,  if  applied  to  armadillo  quadruplets, 
would  lead  to  grave  error.  It  is  probably  safer  to  say 
that  som€  monozygotic  human  twins,  like  some  sets 
of  armadillo  quadruplets,  are  nearly  identical,  while 
others,  hke  various  quadruplet  sets,  may  dilTer  materially 
from  each  other. 

To  me  it  appears  almost  certain  that  Wilder's  twins, 
Nos.  VII,  X,  and  XIII,  are  monozygotic  (duplicates), 


172  THE  BIOLOGY  OF  TWINS 

although  they  show  differences  in  the  presence  of 
certain  friction-ridge  patterns.  It  is  interesting  to 
note  the  doubt  in  Wilder's  mind  as  to  the  nature  of 
these  twins. 

Of  set  VII,  he  says: 

This  case  has  caused  me  considerable  trouble,  owing  to  a 
preconceived  notion  that  the  marks  ought  to  be  found  identical. 
The  family  emphasized  the  facial  resemblance  of  these  twins,  and 
when  I  iirst  saw  them  they  certainly  looked  alike.  One  was, 
however,  an  inch  taller  than  the  other,  and  the  facial  resemblance 
after  a  short  acquaintance  did  not  seem  as  great.  Upon  unpre- 
judiced comparison  the  prints  of  the  palms  are  very  different, 
and  not  at  all  as  in  the  case  of  true  duplicates.  The  finger  pat- 
terns also  do  not  at  all  correspond.  The  sole  markings  are  similar 
but  not  identical.  The  case  is  plainly  one  of  fraternal  twins  that 
resemble  one  another  somewhat  more  than  the  average. 

In  this  connection  let  us  recall,  for  a  moment,  the 
case  of  the  conjoined  twins,  Margaret  and  Mary,  de- 
scribed by  Wilder  in  a  later  paper.  In  that  case  the 
palm-prints  were  nearly  identical,  but  the  right  sole  of 
Mary  was  totally  different  from  her  left  and  from  either 
sole  of  Margaret.  If  the  criteria  employed  for  set  VII 
were  apphed  to  them,  the  twins  Margaret  and  Mary 
would  be  excluded  from  the  category  of  dupHcates;  yet 
there  can  be  no  question  as  to  their  monozygotic  origin. 

It  must  be  emphasized  also  that  a  difference  of  one 
inch  in  height  in  fifteen-year-old  girls  cannot  be  con- 
sidered as  evidence  of  dizygotic  origin;  in  armadillo 
quadruplets  there  are  frequently  much  greater  discrep- 
ancies in  size  than  this.  It  should  furthermore  not  be 
forgotten  that  the  sole  markings  are  similar  in  this 
pair  (Wilder's  VII)  and  would  doubtless  have  caused 


VARIATION  AND  HEREDITY  IN  TWINS        173 

the  twins  to  be  classed  as  identical  had  not  the  palm 
prints  been  found  different. 

Of  twins  No.  X,  Wilder  says: 

These  twins  caused  me  some  little  dirticulty,  although  they 
show  by  the  formulae  great  differences  and  determine  the  set 
as  fraternal  beyond  a  doubt.  The  subjects  are  little  girls  of 
ten,  whom  I  have  seen  but  once,  and  at  the  time  I  took  it  for 
granted  that  they  were  duplicates,  and  as  they  came  to  my 
laboratory  hand-in-hand,  dressed  exactly  alike,  and  each  with 
her  hair  in  two  small  braids,  they  were  certainly  similar,  but  to 
my  assistant  they  did  not  appeal  in  the  same  way,  and  she  judged 
them  fraternal  before  seeing  the  prints.  There  is  a  noticeable 
difference  in  height  and  quite  a  little  in  weight,  greater  than  is 
usually  found  in  true  duplicates. 

Again,  it  is  highly  probable  that  this  pair  is  mono- 
zygotic, although  there  is  a  variation  in  the  distribution 
of  patterns  in  the  two  individuals.  Even  an  examina- 
tion of  finger-prints  reveals  a  very  close  similarity,  the 
difference  being  in  the  palms  of  the  right  hands.  Sole- 
prints  are  not  mentioned. 

Of  twins  No.  XIII,  Wilder  says: 

According  to  personal  appearance  these  should  be  duplicates. 
I  have  never  seen  them,  but  the  one  who  took  the  prints  wrote: 
"The  Misses  ....  are  so  similar  in  coloring  and  features  that 
even  their  best  friends  confuse  them."  It  must  be  confessed,  how- 
ever, that  the  differences  in  the  formulae  cannot  be  reconciled, 
and  that  the  palms  are,  and  remain,  in  respect  to  the  main  lines, 
very  different.  They  both  possess,  however,  certain  peculiar 
markings  in  common,  as  the  thenar  patterns  in  the  left  hands,  or 
the  hypothenar  convergence  in  the  right  hands,  facts  which  would 
help  matters  out  were  there  any  hope  of  reconciling  the  lines.  I 
must  leave  this  as  a  totally  aberrant  case  and  treat  it  as  such 
in  the  summary  given  below.  In  the  other  two  cases  that  have 
caused  trouble,  Nos.  VII  and  X,  the  resemblance  is  not  so  striking 


174  THE  BIOLOGY  OF  TWINS 

and  there  are  marked  differences  in  height  and  weight.  It  will  be 
noted  that  in  these  there  is  a  complete  lack  of  the  bilateral 
symmetry  in  the  hands  of  one  individual,  which  is  usual,  though 
not  invariably  the  case,  in  undoubted  duplicates.  Were  the 
theory  established  beyond  a  doubt,  I  should  unhesitatingly 
diagnose  this  as  a  case  of  fraternals  in  whom  there  happens  to  be 
striking  resemblance,  but  as  one  cannot  be  dogmatic,  I  must 
leave  it  as  recorded,  without  explanation.  The  finger  prints 
correspond  exactly  in  the  two  individuals,  even  more  than  is 
usual  in  those  twins  that  are  unquestionably  duplicates,  yet  it 
will  be  noted  that  they  are,  in  the  main,  ulnar  loops,  the  common- 
est type  of  pattern. 

This,  in  my  opinion,  is  unquestionably  a  case  of 
duplicates  and  exhibits  conditions  quite  parallel  to 
those  shovv^n  by  armadillo  quadruplets. 

These  rather  long  but  significant  quotations  serve  to 
show  the  difficulty  of  applying,  as  a  criterion  of  mono- 
zygotic origin  of  twins,  resemblances  or  lack  of  resem- 
blances in  any  unit  characters.  It  would  seem,  on  the 
whole,  more  feasible  to  trust  to  one's  judgment  of  the 
general  similarity  in  features,  coloring,  disposition,  and 
the  like,  for  such  resemblances  are  at  least  as  important 
elements  in  the  personality  as  are  finger-prints;  a 
general  summation  of  resemblances  is  more  likely  to 
be  a  sound  basis  than  any  single  detail  could  be,  espe- 
cially since  we  know  that  monozygotic  armadillo 
quadruplets  often  differ  markedly  among  themselves 
in  respect  to  characters  of  strictly  comparable  nature. 
The  presence  of  any  type  of  symmetry  reversal  would 
to  my  mind  outweigh  any  lack  of  detailed  resemblance 
in  deciding  that  any  given  set  of  twins  is  monozygotic. 

Whether  in  the  light  of  these  circumstances  Wilder's 
idea  is  justifiable — that  we  can  measure  the  limits  of 


VARIATION  AND  HEREDITY  IN  TWINS        175 

hereditary  control  in  twins  by  concluding  that  the 
general  configurations  of  friction-skin  patterns  is  pre- 
determined and  only  the  minutiae  are  beyond  the  limits 
of  hereditary  control — is  a  question  which  will  have 
suggested  itself  to  the  reader  before  this.  This  con- 
clusion was  based  on  those  cases  of  duplicates  which 
were  alike  in  the  general  configuration  of  the  patterns; 
but  if,  as  seems  certain,  monozygotic  twins  sometimes 
differ  not  only  in  minutiae  but  in  the  general  con- 
figuration of  friction  ridges,  this  conclusion  as  to  the 
limits  of  hereditary  control  fails  to  hold. 

It  certainly  fails  to  hold  for  armadillo  quadruplets, 
which  are  uniformly  monozygotic. 

HEREDITARY  CONTROL  AND  SOMATIC  SEGREGATION 

To  find  a  failure  of  bilaterality  in  certain  friction- 
ridge  patterns  in  a  single  individual  is  not  at  all  unusual. 
For  example,  when  I  examine  the  friction  patterns  of 
my  own  hands,  I  find  a  pronounced  dissimilarity  in  the 
two.  The  right  hand  has  a  very  conspicuous  h^-pothenar 
whorl  not  even  suggested  in  the  left.  In  the  left  hand 
there  occurs  a  well-defined  triradius,  absent  in  the  right. 
In  other  respects  the  two  hands  are  similar  but  not 
identical.  If  hereditary  control  is  to  be  tested  by  a 
comparison  of  duplicate  twins,  which  are  believed  to  be 
monozygotic,  why  would  it  not  be  much  simpler  to 
test  it  by  a  comparison  of  the  antimcric  halves  of  a 
single  individual  who  is  known  to  be  monozN'gotic  ? 
Unquestionably,  if  by  hereditary  control  is  meant 
that  identity  of  two  or  more  homologous  or  bilateral 
products  of  a  single  zygote  is  predetermined,  even  the 
main  patterns  of  friction  ridges  cannot  be  said  to  be 


176  THE  BIOLOGY  OF  TWINS 

hereditarily  controlled  in  the  same  way  on  both  sides  of 
the  body  of  those  individuals  who  are  bilaterally  asym- 
metrical. 

The  unilateral  appearance  of  an  inherited  unit 
character,  such  as  a  friction-skin  pattern,  almost 
certainly  implies  some  unilaterality  in  the  somatic 
distribution  of  the  differentiating  factor  for  this  charac- 
ter. Whether  the  character  appears  in  one  or  in  both 
of  a  pair  of  twins  (which  are  genetically  equivalent  to 
the  right  and  left  sides  of  a  single  individual),  or,  finally, 
whether  it  appears  in  one,  two,  three,  or  four  members 
of  a  set  of  armadillo  quadruplets,  depends  on  whether 
the  differentiating  factor  is  distributed  during  the 
earliest  cleavage  in  a  unilateral  or  bilateral  fashion;  in 
other  words,  whether,  with  respect  to  the  differen- 
tiating factor  in  question,  the  earliest  cleavages  have 
been  equational  or  differential.  All  of  the  cells  de- 
rived from  the  blastomere  that  receives  the  factor 
will  produce  individuals  with  the  character,  unless,  as 
often  happens,  subsequent  cleavages  still  further  limit 
the  distribution  of  the  factor  by  repeated  differential 
division. 

Thus  we  appear  to  have  the  possibility  of  a  segrega- 
tive mechanism,  which,  in  so  far  as  an  individual  or 
set  of  monozygotic  twins  is  concerned,  might  give 
results  that  would  resemble  the  segregation  of  unit 
characters  in  the  maturation  division  of  the  germ-cells. 
That  segregation  of  unit  characters  resulting  in  the  so- 
called  purity  of  gametes  probably  has  its  counterpart 
in  the  segregations  that  occur  in  the  early  cleavages 
in  the  armadillo  and  is  not  confined  to  the  gonads 
seems  certain;  there  appears  to  be  a  parallel  segregation 


VARIATION  AND  HEREDITY  IN  TWINS        177 

in  the  germ-plasm  associated  with  the  soma  of  each 
individual.  Within  a  single  set  of  armadillo  quad- 
ruplets one  individual  having  a  unit  character  (a  dou- 
bhng  of  integumentary  units)  in  the  soma  transmits  this 
character  to  its  offspring,  while  another  individual 
(derived  from  the  same  zygote)  lacking  this  character 
in  the  soma  fails  to  transmit  it  to  its  offspring.  In 
conclusion,  I  should  therefore  like  to  emphasize  the 
fact  that  somatic  divisions  may  be  as  important  agents 
in  segregating  unit  characters  as  germinal  divisions 
involved  in  the  formation  of  gametes  (maturation  or 
reduction  divisions)  are  beheved  to  be.  On  this  theory 
it  might  well  happen  during  cleavage  that  cells  derived 
from  one  part  of  a  single  gonad  would,  prior  to  matu- 
ration, contain  certain  determiners  which  others  in 
another  part  of  the  same  gonad  lack. 

It  would  appear,  then,  that  the  hmits  of  hereditary 
control  are  not  to  be  measured  by  a  comparison  of 
twins,  or  even  by  comparing  the  antimeric  halves  of 
the  same  individual.  The  whole  question  hinges  on  the 
equahty  or  inequality  of  distribution  during  cleavages 
of  the  determinative  factors;  this  involves  what  we 
have  called  somatic  segregation. 

STATISTICAL   METHODS    OF   DETERMINING   THE    LIMITS    OF 

HEREDITARY   CONTROL 

Another  more  rehable  method  of  testing  the  limits 
of  hereditary  control  than  those  applied  to  individual 
cases  is  the  statistical  method  which  applies  to  large 
groups.  We  can  find  out  how  strong  on  the  average  is 
the  hereditary  control  exercised  by  the  predeterminative 
mechanism  of  the  germ-plasm  with  respect  to  certain 


178  THE  BIOLOGY  OF  TWINS 

measurable  or  enumerable  characters.  In  human  twins 
only  dimensional  differences  have  been  determined, 
and  these  are  subject  to  too  large  an  element  of  environ- 
mental control  to  be  used  as  a  test  of  heredity. 

In  the  armadillo,  however,  we  find  an  ideal  material 
for  statistical  study  in  the  armor  scutes  in  the  nine 
bands  of  armor  and  in  the  armor  shields.  A  determina- 
tion of  the  coefhcient  of  correlation  for  large  numbers 
of  sets  of  quadruplets  reveals  the  fact  that  on  the 
average  this  coefficient  approaches  within  about  7  per 
cent  of  complete  identity.  For  example,  the  coefficient 
of  correlation  derived  from  an  array  of  56  sets  of  male 
quadruplets  gives  a  coefficient  of  o. 9294=1=0.0057.  A 
similar  high  coefficient  was  found  for  59  sets  of  female 
quadruplets.  These  figures  were  derived  from  a  study 
of  the  total  numbers  of  scutes  in  the  nine  bands.  Now, 
when  we  compare  with  this  the  coefficients  of  correlation 
for  individual  bands,  it  is  found  that  there  is  a  very 
decided  drop  from  the  near  approach  to  identity  seen 
in  the  banded  region  as  a  whole,  for  the  average  coeffi- 
cient of  correlation  determined  for  three  sample  bands 
(i,  5,  and  9)  is  about  0.4+.  These  results  may  be 
interpreted  as  showing  that  the  total  number  of  scutes 
in  a  large  armor  region  is  rather  definitely  predeter- 
mined, but  the  alignment  of  the  scutes  into  rows  or 
bands  is  a  process  involving  developmental  mechanics 
of  a  cruder  sort  which  appears  to  be  largely  beyond 
the  limits  of  hereditary  control.  Here  then  we  would 
appear  to  be  in  possession  of  facts  that  should  enable  us 
to  draw  the  line  between  "nature  and  nurture"  or  to 
determine  the  limits  of  hereditary  control.  But,  as 
is  usual  with  statistical  results,   the  averaging  up  of 


VARIATION  AND  HEREDITY  IX  TWINS        179 

figures  conceals  the  fundamental  facts.  Studies  in  the 
heredity  of  armor  units  reveal  not  infrequently  a 
situation  in  which,  so  far  as  individual  bands  are  con- 
cerned, there  is  a  close  approach  to  identity  between 
mother  and  offspring;  in  other  bands,  however,  there 
may  be  a  pronounced  lack  of  heredity.  In  some  cases, 
moreover,  one  offspring  in  a  set  is  almost  identical, 
band  for  band,  with  the  mother,  while  another  offspring 
of  the  same  set  shows  marked  differences  from  the 
mother.  Once  more  then  we  shall  have  to  call  upon 
the  mechanism  of  somatic  segregation,  which  is  respon- 
sible for  the  segregation  of  biparental  units,  so  that  one 
individual  of  a  monozygotic  set  shows  the  maternal 
scute  count  and  another  shows  a  very  different  scute 
count,  presumably  like  that  of  the  father. 

Hence,  although  a  coefiicient  of  correlation  derived 
by  averaging  the  conditions  in  a  large  number  of  mono- 
zygotic sets  of  offspring  may  be  a  valuable  measure  of 
the  average  performance  of  the  species,  it  has  no  value 
when  appHed  to  individual  cases.  One  must  conclude, 
therefore,  that  no  definite  law  is  to  be  posited  as  to  the 
relative  potency  of  ''nature  and  nurture"  or  of  the  pre- 
determinative  versus  the  epigenetic  factors  of  develop- 
ment. Every  character  evidently  has  a  genetic  basis  in 
the  zygote,  but  the  exact  expression  of  the  character 
is  dependent  upon  developmental  or  epigenetic  factors 
that  vary  in  each  individual  case.  There  appears  no 
longer  to  be  any  point  to  an  attempt  to  determine  the 
relative  potency  of  predetemiinative  and  epigenetic 
factors  in  development. 


INDEXES 


INDEX  OF  SUBJECTS 


Note. — References  give  the  number  of  the  page  on  which  the  term  is  first  defined 
or  on  which  the  consideration  of  the  subject  begins.  Sometimes,  when  the  topic  occu- 
pies several  pages,  the  first  and  last  pages  are  given.  When  a  term  is  used  a  great  many 
times  or  when  a  species  is  referred  to  repeatedly  throughout  the  book,  only  some  of 
the  more  important  references  are  given. 


Amnion:  amniotic  connecting 
canals,  52,  65;  common  amnion 
in  Dasypus,  51-57;  origin  of,  in 
Das y pus,  46. 

Annadillo:  armor  of,  27;  corpus 
luteum  of,  32;  food  of,  29; 
habits  of,  28;  hairy  armadillo 
(see  Peludo),  77-83;  mating  of, 
29;  Miilita  armadillo,  69-77; 
nine-banded  armadillo  of  Texas, 
27;  ovaries  of,  32;  ovogenesis 
of,  32;  quadruplets,  16,  24; 
uterus  of,  30. 

Atretic  follicles,  30,  35. 

Bilateral:  development,  5;  divi- 
sion, 5;  doubling,  5;  struc- 
tures,   5. 

Blastocyst,  i.     See  also  Egg. 

Blastodermic  vesicle,  i,  12,  38. 
See  also  Egg. 

Blastotomy,  91. 

Chromosomes  in  Dasypus,  36. 
Corpus  luteum  (corpora  lutea),  11, 

32. 
Cosmohia,  19-21. 
Co-twin,  6. 
Cyclopic:  eyes,  5;   monsters,  5. 

Dasypus:  gymnurus,  83;  hybridus, 
25,  118;  novcmclnclus,  2,  6,  25, 
118;  sexcinclus,  83. 

Dasyurus,  34,  36,  37,  42. 

Deutoplasm:  extrusion  in  arma- 
dillo egg,  35;  zone,  33. 


Diplopagi,  18,  23. 
Diplotrophoblast,  45,  53. 
Discus  proligerus,  33. 
Dizygotic,  3. 

Egg:  binucleated,  16;  eutherian, 
40;  use  of  word  as  equivalent  of 
blastocyst,  etc.,  i. 

Embr>'ology :  of  Dasypus  hybridus, 
69-77;  of  D.  novcmcinctus,  39- 
67;  of  Euphractus  villosus,  77- 
83;  of  man,  17,  23. 

Embryos:  earliest  of  Dasypus,  40; 
number  of  in  D. ,  hybridus,  69; 
of  ferret,  92;  of  Lupus,  92; 
of  mouse,  42,  92;  of  sheep,  92; 
orientation  of  in  uterus  of 
Dasypus,  61;  pairing  of  in 
Dasypus,  58;  primary  cmbr>os 
in  Dasypus,  47;  rudimcntar>'  in 
D.  hybridus,  75,  76;  secondary 
embryos  in  Dasypus,  49. 

Euphractus  villosus,  3,  6,  77-83, 
108,  118. 

Extra-embryonic   cavity,   44.   46, 

47- 

Fertilization  of  Dasypus  novcm- 
cinctus, 36,  37. 

Formative  zone,  ^s. 

Freemartin:  blood  admixture  in. 
104;  fertile,  96;  hormone  the- 
ory of  origin  of,  104. 


Gastrulation     in     armadillo    egg, 
40-42. 


183 


1 84 


THE  BIOLOGY  OF  TWINS 


Germinal  vesicle  of  armadillo  egg, 

34. 

Germ-layer  inversion,  42,  108. 

Hereditary  control,  limits  of,  6, 
175- 

Heredity:  in  armadillo  quad- 
ruplets, 125-55;  in  human 
twins,  155-69;  of  double  bands, 
133-45;  of  double  scutes,  145- 
49;  of  numbers  of  scutes,  145- 
49;   versus  environment,  169. 

Hermaphrodite,  96,  97. 

Heterosexual,  2,  100. 

Homosexual,  2,  100. 

Inner-cell  mass,  40. 

Inversion  of  germ  layers,  42,  108. 

Litomastix,  112. 

Marsupial  cat   {Dasyurus),  42. 
Mataco  {Tolypeutes),  83. 
Mirror-image  (mirror-imaging),  4, 

17,  152-55- 
Mon-amniotic,  23. 
Monozygotic,  3,6. 

Monsters:    cyclopic,     5;    double, 

4,8. 

Mulita  {Dasypus  hybrldus),  27, 
65,  68. 

Multiple  births,  4,  113. 

Ovocyte  of  armadillo,  t,^. 
Ovogenesis  of  armadillo,  32-35. 

Pairing:  exceptions  to  pairing  in 
armadillo  quadruplets,  62;  of 
embryos   in  same,  58. 

Parasite  (?)  of  armadillo  egg,  86. 

Parthenogenesis  in  armadillo  egg, 
30,  36. 

Peludo  {Euphractus  viUosus),  77- 
83. 

Placenta:  the  development  of  in 
Dasypus,  66;  discoid  of  Dasy- 
pus, 58-60;    primitive  (Trager) 


of  Dasypus,  44;    secondary  of 
Dasypus,  56. 

Placentation  area,  31. 

Polarity :  reversal  of,  in  armadillo 
ovocyte,  35. 

Polyembr>'ony :  in  hymenoptera, 
112;  specific,  3,  25,  39,  84;  the- 
ories of,  85-92. 

Putorius  (ferret),  91. 

Quadruplets:  armadillo,  3,  4, 
53,  60,  63,  171;  human,  3,  114. 

Quiescence,  period  of,  39,  86. 

Reversal:  of  polarity  in  arma- 
dillo ovocyte,  35;  of  primary 
axis  of  armadillo  egg,  44;  of 
symmetry  in  armadillo  quad- 
ruplets, 141,  145,  148,  153, .154; 
of  symmetry  in  human  duplicate 
twins,  15,  154,  159,  160,  164, 
165. 

Sex:  determination  of,  6,  no,  in, 
112;  differentiation  of,  no, 
116,  117;  ratios  in  twins  and 
multiple   births,    10,    no,    113, 

115- 
Somatic  segregation,  149-52,  174- 

77.. 

Tatu  (Dasypus),  25. 

Tatusia  (Dasypus),  25. 

Tolypeutes,  28,  83. 

Trager,  45-54- 

Trager  cavity,  69. 

Triplets:    human,  3;    calves,  108. 

Trophoblast,  40,  43,  44. 

Twinning:  blastotomy  theory  of, 
16,  17,  89;  budding  theory  of, 
73,  89;  fission  theory  of,  92; 
hereditary  twinning,  118-20. 

Twins:  autosite  and  parasite,  18; 
brain  convolutions  of,  166; 
conjoined,  3,  5,  8,  18,  20; 
craniopagi,  19;  dizygotic,  15; 
double  monsters,  3,  9,  14,  21; 


INDEXES 


>85 


duplicate,  5,  8,  14,  15;  finger 
prints  of,  160;  fraternal,  8,  15; 
friction  skin  patterns  of,  155- 
69;  hair  whorls  of,  166; 
human,  7,  8;  identical,  6; 
in  cattle,  96;  in  deer,  95;  in 
horses,  95;  in  Mulila  {Dasypiis 
hybrid Hs\,  77-83;  in  Peludo, 
69-77;  in  ruminants,  95-109; 
in  sheep,  92,  96,  114,  119-23; 
in  swine,  95;  intra-uterine 
relations  of,  10,  23;  literar}-, 
8;  look-alike,  8;  mental  traits 
of,  168;  monochoreal,*  6,  23; 
monozygotic,  9,    15;    opposite- 


scxcd,  2;  palm  and  sole  pal- 
terns  of,  i55-()g;  pygopagi,  19; 
same-sexed,  2;  sheep  embryo, 
92;  Siamese,  3,  7,  8,  18;  thora- 
copagi,  19;   ungulate,  32. 

Uterus,  human.    10;    of  Dasypus 

novemciiiclus,  30. 

Variation  and  heredity:   in  arma 
dillo   quadruplets,    123-55;     in 
human  twins,  155-69. 

Zona  pellucida,  ^s- 
Zygote,  3. 


INDEX  OF  AUTHORS 


Assheton,  R.,  91-93- 

Barnum,  P.  J.,  18. 
Bateson,  W.,  100,  154. 
Bell,  A.  G.,  119-22. 
Berger,  P.,  123. 
Blundell,  21. 

Carson,  R.  D.,  83. 
Chapman,  H.  C,  83. 
Chidester,  F.  E.,  168. 
Child,  C.  M.,  87. 
Cole,  L.  J.,  100,  122. 

Danforth,  C.  H.,  123. 

Fernandez,  M.,  27,  38,  53,  65,  68- 

83. 
Fischer,  22,  23. 

Galton,  F.,  155,  169. 

Hart,  D.  B.,  99. 
Hill,  J.  P.,  35,  42. 
Hunter,  J.,  96. 

Jhering,  H.  von,  26. 

Kolliker,  A.  von,  12,  16,  83. 


Lillie,  F.  R.,  7,  102,  108. 

Mellisinos,  K.,  40. 

Newman,  H.  H.,  17,  29,  30,  32,  36, 
126,  130. 

Newman,  H.  H.,  and  J.  T.  Patter- 
son, 16,  27,  38,  53. 

Nichols,  J.  B.,  9,  10,  114. 

Numan,  A.,  97. 

Osgoode,  W.  H.,  7. 

Patterson,  J.  T.,  31,  39,  40,  50-52. 
Pearl,  R.,  108,  114,  122. 

Rosner,  M.  A.,  26. 

Sano,  F.,  166,  167. 
Schultze,  O.,  II,  15. 
Silvestri,  F.,  112. 
Spiegelberg,  0.,  98,  99. 
Strahle,  H.  von,  60. 

Toda,  K.,  7,  27,  40. 

Wentworth,  E.  N.,  11 4-1 5. 
Wilder,  H.  H.,  9,  11,  16,  22,  155- 
69. 


PROPERTY  LIBRARY 

n.  C.  State  College 


186 


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